Does my UK limited company need a US EIN to file Form 5471?

No. The Instructions for Form 5471 at irs.gov/instructions/i5471 require a reference ID number in item 1b(2) only where no EIN was entered in item 1b(1). A UK company with no US filing, payroll or withholding role generally never needs an EIN, and the reference ID number exists precisely for that situation. The number is established by or on behalf of the US person filing the form, and no application to the IRS is required.

What format must a Form 5471 reference ID number take?

It must be alphanumeric, meaning alphabetical, numeric, or any combination of the two. No special characters and no spaces are permitted, so hyphens, slashes and full stops are out. The length is limited to 50 characters. The number must uniquely identify the foreign entity and must be used consistently from tax year to tax year for that company. FOREIGNUS and APPLIED FOR are no longer permitted entries.

Can I use my Companies House company registration number as the reference ID?

There is no IRS rule on this, and it should not be presented as one. As a practical matter a UK registration number is already alphanumeric with no spaces or special characters, is unique across the register, survives a change of company name, and can be verified later at find-and-update.company-information.service.gov.uk. Our own convention is a fixed prefix plus the registration number, for example UK09876543, but any compliant identifier is acceptable to the IRS.

What do I enter if the reference ID number changes?

Enter the new reference ID number in item 1b(2) and the previous reference ID number or numbers in item 1b(3), separating multiple prior numbers with a space. The Form 5471 instructions state this correlation applies only to the first year the new number is used, and only on page 1 items 1b(2) and 1b(3). On every separate schedule, enter only the current reference ID number.

If several late Forms 5471 are filed together, can each year use a different number?

No. Each Form 5471 covers a separate annual accounting period, and the instructions require the same reference ID number to be used consistently from tax year to tax year for a given foreign corporation. Preparing the years simultaneously does not merge them. Fix the identifier before drafting the earliest year, use it on every year in the package and on the following year's return, and leave item 1b(3) blank where no earlier Form 5471 was ever filed.

Does the same reference ID have to appear on Form 8858 and Form 8992?

Yes, where those forms apply. The Form 5471 instructions require the reference ID number to be entered on Form 8858, line 3c(2), where the foreign corporation is the tax owner of a foreign disregarded entity or foreign branch, and require the number on Schedule A or Schedule B of Form 8992 for a CFC to be the same as the number on Form 5471, item 1b(2). The Form 8858 and Form 8992 instructions state the requirement from their own side.

Form 5471 Reference ID Numbers for UK Companies

Form 5471 requires a reference ID number in item 1b(2) for a UK limited company that has never held a US employer identification number, and that number is one the filer creates rather than one the IRS issues. The Instructions for Form 5471, revision 12-2025, published at irs.gov/instructions/i5471, state that a reference ID number is required in item 1b(2) only in cases where no EIN was entered in item 1b(1) for the foreign corporation, and that filers are permitted to enter both. The requirements are short: the entry must be alphanumeric, no special characters or spaces are permitted, and the length of a given reference ID number is limited to 50 characters. Because reference ID numbers are established by or on behalf of the US person filing the form, the instructions confirm there is no need to apply to the IRS to request one or for permission to use it.

The rule that causes the damage is not the format rule. It is the durability rule. The instructions state that the same reference ID number must be used consistently from tax year to tax year with respect to a given foreign corporation, and that if a reference ID number falls out of use, for example because the foreign corporation no longer exists due to disposition or liquidation, that number cannot be used again for another foreign corporation for purposes of Form 5471 reporting. A reference ID number is therefore not a box to be filled in each spring. It is a permanent identifier, chosen once, that has to survive name changes, share transfers, a change of accountant, an entity classification election and, in the catch-up cases we see most often, a stack of delinquent years all prepared on the same afternoon.

This article works through the identification block at the top of page 1 of Form 5471 as it applies to a UK company on the register at Companies House with no US employer identification number: when an EIN is required instead, the exact rules for the reference ID number, what to do when it changes, which related forms have to carry the same number, and the address, country, activity and currency fields that sit alongside it. It then sets out the naming convention we use for UK groups, which is our own practice and not an IRS rule, and the two failure modes that account for almost every reference ID problem that lands on our desk: an identifier improvised in year one, and a multi-year catch-up filing where the years drift apart.

Does a UK company need an EIN, or a Form 5471 reference ID number?

The hierarchy on page 1 is straightforward. Item 1b(1) asks for the employer identification number of the foreign corporation, if any. Item 1b(2) asks for a reference ID number. The instructions at irs.gov/instructions/i5471 make item 1b(2) mandatory only where item 1b(1) is empty. If the UK company holds an EIN, the EIN goes in 1b(1) and that is sufficient. If it does not, a reference ID number must be entered, and it must uniquely identify that foreign entity.

Most UK trading and holding companies owned by a US person never acquire an EIN, because nothing in their history requires one. They have no US payroll, no US filing obligation of their own, and no US withholding role. The reference ID route exists precisely for them. The situations where a UK company does end up with a US employer identification number, and therefore uses item 1b(1) rather than 1b(2), are narrower than owners expect.

An entity classification election is made on the company's behalf on Form 8832. The Form 5471 instructions point to Regulations section 301.6109-1(b)(2)(v), which requires the foreign corporation to have an EIN for that election.
The company itself has a US federal filing or reporting obligation in its own name, so the return it files requires an identifying number.
The company is a withholding agent or payer that has to be identified on US information reporting.
The company has US employees or a US payroll registration.
An EIN was obtained historically, often at the point of a US bank or brokerage onboarding, and then sat unused. It still belongs in item 1b(1) once it exists.

Where an EIN is needed, note that the online application route is closed to a UK company. The IRS application page at irs.gov/businesses/small-businesses-self-employed/apply-for-an-employer-identification-number-ein-online lists a principal place of business in the US or US territories as a condition of using the tool, and directs applicants whose principal place of business is outside the US to apply by phone, fax or mail. The Instructions for Form SS-4, revision 12-2025, at irs.gov/instructions/iss4, state that an applicant with no legal residence, principal place of business, or principal office or agency in the United States or US territories may call 267-941-1099, which is not a toll-free number, from 6:00 a.m. to 11:00 p.m. Eastern time, Monday through Friday, to obtain an EIN. The same instructions give fax numbers of 855-215-1627 from within the United States and 304-707-9471 from outside it.

There is one detail worth holding on to before the practical sections below. The instructions permit a filer to enter both an EIN in item 1b(1) and a reference ID number in item 1b(2). That permission is not decorative. Where a UK company acquires an EIN part-way through a filing history, keeping the original reference ID number in 1b(2) alongside the new EIN is the cleanest way to preserve the thread back to the earlier years.

What are the rules for a Form 5471 reference ID number?

The instructions define a reference ID number as a number established by or on behalf of the US person identified at the top of page 1 of the form that is assigned to a foreign corporation with respect to which Form 5471 reporting is required, and state that these numbers are used to uniquely identify the foreign corporation in order to keep track of the corporation from tax year to tax year. The requirements themselves, taken directly from the Instructions for Form 5471 at irs.gov/instructions/i5471 and the printable version at irs.gov/pub/irs-pdf/i5471.pdf, are these.

The entry must be alphanumeric. The instructions define alphanumeric for this purpose as meaning the entry can be alphabetical, numeric, or any combination of the two.
No special characters are permitted. Hyphens, slashes, full stops, ampersands and underscores are all out.
No spaces are permitted within a reference ID number.
The length of a given reference ID number is limited to 50 characters.
The number must uniquely identify the foreign entity.
The same reference ID number must be used consistently from tax year to tax year with respect to a given foreign corporation.
If a reference ID number falls out of use, for example because the foreign corporation no longer exists due to disposition or liquidation, it cannot be used again for another foreign corporation for purposes of Form 5471 reporting.
No application to the IRS is needed. The filer establishes the number, and no IRS permission is required to use it.

Fifty characters is a generous ceiling, and it is worth saying plainly that nobody should be anywhere near it. A reference ID number is not a description. It is a key. The schemes that survive are short, mechanical and impossible to mistype twice in the same way. The schemes that fail are the ones that tried to carry meaning about the company inside the identifier, because meaning changes and identifiers must not.

One further scoping point. The instructions state that the reference ID number assigned to a foreign corporation on Form 5471 generally has relevance only on Form 5471, its schedules, and any other form that is attached to or associated with Form 5471, and generally should not be used with respect to that foreign corporation on any other IRS forms. The exceptions to that scoping rule are specific and are covered below.

Why FOREIGNUS and APPLIED FOR are no longer permitted

Older Forms 5471, and a good deal of software behaviour and published guidance that still circulates, relied on placeholder strings. The Instructions for Form 5471 now close that off in one sentence: taxpayers no longer have the option of entering FOREIGNUS or APPLIED FOR in a column that requests an EIN or reference ID number with respect to a foreign entity, and instead, if the foreign entity does not have an EIN, the taxpayer must enter a reference ID number that uniquely identifies the foreign entity.

The same tightening runs through the related forms. The Instructions for Form 8992, revision 12-2024, at irs.gov/instructions/i8992, state that with respect to each CFC reported on Schedule A, if the CFC has an EIN you must enter that EIN in column (b), that APPLIED FOR must not be entered in column (b) in lieu of an EIN, and that if the CFC does not have an EIN you must enter a reference ID number that uniquely identifies the CFC.

The reason for the change is mechanical rather than punitive. A placeholder is not an identifier. If every UK company owned by every US filer carries the same string, nothing distinguishes one from another and nothing links a company to its own prior year. In the returns we prepare, the appearance of FOREIGNUS or APPLIED FOR in a prior-year Form 5471 is a reliable signal that the earlier years were prepared without a durable identification scheme at all, and that a reference ID has to be established and, where an earlier reference ID did exist, correlated forward.

What happens when a Form 5471 reference ID number changes?

Item 1b(3) on page 1 is headed previous reference ID number(s), if any. It exists so that a change of identifier does not break the chain. The instructions set out the mechanics precisely: enter the new reference ID number in item 1b(2) and enter the previous reference ID number or numbers in item 1b(3), and where there is more than one old reference ID number, enter a space between each such number. The 50-character limit and the alphanumeric and no-special-characters rules apply to those entries too.

Two limits on that correlation are easy to miss and both matter. First, the instructions state that the correlation requirement applies only to the first year the new reference ID number is used. Second, they state that it applies only on Form 5471, page 1, items 1b(2) and 1b(3), and that on all separate schedules for Form 5471 the filer should enter only the current reference ID number in the applicable entry space. In other words, there is exactly one filing in which the link between the old and the new identifier is visible to the IRS. Miss it, and there is no later form on which to put it right.

The instructions give examples of situations that warrant correlating a new reference ID number with a previous one.

A merger or acquisition. The instructions state that a Form 5471 filer must use a reference ID number that correlates the previous reference ID number with the new reference ID number assigned to the foreign corporation. In practice this is the case where a US acquirer takes on a UK company that a different US filer was already reporting under a different identifier.
An entity classification election made on behalf of a foreign corporation on Form 8832. Because Regulations section 301.6109-1(b)(2)(v) requires the foreign corporation to have an EIN for that election, the instructions direct that for the first year Form 5471 is filed after the election, the new EIN must be entered in item 1b(1) and the old reference ID number must be entered in item 1b(2).
In subsequent years after such an election, the instructions permit the filer to continue entering both the EIN in item 1b(1) and the reference ID number in item 1b(2), but require at least the EIN in item 1b(1).

Read the Form 8832 sequence carefully, because it is the one preparers get backwards. In the first post-election year the old reference ID number goes into item 1b(2), the current reference ID field, not into item 1b(3). The instructions place it there deliberately so that the EIN and the historic identifier appear side by side on the same page. Item 1b(3) is for a previous reference ID number that has been replaced by a different reference ID number, not for a reference ID number that has been supplemented by an EIN.

Which other forms must carry the same reference ID number?

The general scoping rule is that the reference ID number belongs to Form 5471 and its schedules. The Form 5471 instructions then carve out two cross-form requirements, and both are common in UK structures.

Form 8858. The instructions state that the foreign corporation's reference ID number should also be entered on Form 8858 if the foreign corporation is listed as a tax owner of a foreign disregarded entity or foreign branch, and refer the filer to the instructions for Form 8858, line 3c(2). The Instructions for Form 8858, revision 12-2024, at irs.gov/instructions/i8858, confirm that a reference ID number is required on line 3c(2) only in cases where no EIN was entered on line 3c(1), and that where the tax owner is a CFC for which Schedule A or Schedule B of Form 8992 is required, the reference ID number entered on line 3c(2) must be the same as the reference ID number entered on Form 5471, item 1b(2).
Form 8992. The instructions state that if a US shareholder is required to file Schedule A (Form 8992) or Schedule B (Form 8992) with respect to the CFC, the reference ID number on Form 5471 and the reference ID number used on Schedule A or Schedule B for that CFC must be the same. The Form 8992 instructions repeat the requirement from their own side and point back to the instructions for Form 5471, item 1b(2).

Inside the Form 5471 package itself, the reference ID number recurs on the separate schedules. The schedules that carry a reference ID number of foreign corporation field include Schedule E for income, war profits and excess profits taxes paid or accrued, Schedule H for current earnings and profits, Schedule I-1 for information for global intangible low-taxed income, Schedule J for accumulated earnings and profits, Schedule P for previously taxed earnings and profits of the US shareholder, Schedule Q for the CFC income by CFC income groups, and Schedule R for distributions from a foreign corporation. On every one of those, the entry is the current reference ID number only.

The practical consequence for a UK company owner is that a single identifier has to be correct in a dozen places across three different forms in the same tax return, and then again in every subsequent year. That is exactly the kind of task that survives on convention and fails on memory. Our Form 5471 and GILTI work sits within the corporate compliance service described at us-uktax.com/business-corporate-tax-planning, and the identification block is the first thing set on a new engagement, before any figure is computed.

A reference ID naming convention for a UK group that survives ten years

Everything in this section is our own practice, not an IRS rule. The IRS does not prescribe any naming convention, does not review the identifier you choose, and imposes no requirement whatsoever about UK company registration numbers. What the IRS does require is uniqueness, format compliance and consistency across years, and the convention below exists only because it makes those three things automatic rather than discretionary.

The design brief for a UK group is unusual. A single US shareholder may hold a holding company, two or three trading subsidiaries and a dormant company, some incorporated in England and Wales and some in Scotland, over a period in which the group renames a subsidiary, sells one, incorporates another, changes registered office twice and changes accountant once. The identifier has to be untouched by all of that. The rules we apply are these.

Derive the identifier from something external and immutable. A UK company registration number is issued by Companies House at incorporation and does not change when the company changes its name, its registered office, its directors or its shareholders.
Use a fixed prefix so that the identifier can never be mistaken for a bare registration number, an EIN or an account number when it appears on a schedule out of context. We use UK.
Never use separators. Hyphens, spaces and full stops are special characters or spaces and are not permitted by the Form 5471 instructions.
Keep the whole identifier under twenty characters. The 50-character ceiling is a limit, not a target, and short identifiers are transcribed correctly more often.
Fix the identifier in a single controlled document held with the engagement file, not in the tax software, so that it survives a change of software or a change of preparer.
Record the identifier in the same document alongside the company name at the date of adoption, the registration number, the date the identifier was first used on a filed Form 5471, and the tax years in which it has appeared.
Never reuse an identifier. When a company is sold, struck off or liquidated, mark its identifier as retired in the same document. The instructions prohibit reusing it for another foreign corporation.

The anti-patterns are just as important, because every one of them appears in files we have taken over.

Initials plus a year, such as an identifier built around 2019, which then looks wrong on every subsequent year's form and invites a preparer to update it.
Anything derived from the company's trading name or registered name, which changes.
Anything derived from the shareholder's name or initials, which breaks the moment shares move between family members or into a holding company.
Sequential client codes taken from the preparer's practice management system, which are reused when a client leaves and a new client is onboarded, in direct conflict with the no-reuse rule.
Identifiers containing LTD, LIMITED or PLC, which encode a company status that can change on re-registration.
Identifiers containing the registered office postcode, which changes whenever the company moves or changes formation agent.
Legacy placeholder strings such as FOREIGNUS or APPLIEDFOR, which the instructions no longer permit and which identify nothing.

Can the Companies House number be used in a Form 5471 reference ID number?

There is no IRS rule on this point, and it should not be presented to a client as one. What can be said is that a UK company registration number happens to have exactly the properties a reference ID scheme needs, and that using it as the payload of a self-assigned identifier is a sound practical choice. HMRC's COTAX manual at gov.uk/hmrc-internal-manuals/cotax-manual/com40011 describes the format as up to eight numerals, or normally two alphabetical characters referred to as the prefix followed by up to six alphanumeric characters. In everyday terms, an England and Wales company carries eight digits, a Scottish company carries an SC prefix and six digits, and a Northern Ireland company carries an NI prefix. Limited liability partnerships carry OC, SO or NC prefixes.

That format is already alphanumeric with no special characters and no spaces, which means it drops into a Form 5471 reference ID number without adaptation. It is unique across the whole UK register, so two companies in the same group can never collide. It is verifiable by anyone at any point in the future against the public register through gov.uk/get-information-about-a-company and the search service at find-and-update.company-information.service.gov.uk, which matters when a preparer changes and the new preparer needs to confirm which company a historic identifier refers to. And it is stable across precisely the events that break other schemes, because a UK company keeps its registration number through a change of name.

So the convention we use is a fixed prefix followed by the registration number with nothing between them. A company registered in England and Wales under number 09876543 becomes UK09876543. A Scottish company registered under SC456789 becomes UKSC456789. That is eleven characters at most, alphanumeric, unique, externally verifiable and immune to renaming. To repeat the point, this is our convention. The IRS neither requires it nor recognises it, and a filer who has already established a different compliant identifier for a company should keep using it rather than switch, because switching triggers the correlation requirement in item 1b(3) and there is no advantage in a tidier identifier that costs a broken chain.

Why the identifier improvised in year one is the one that breaks in year six

A reference ID number chosen casually causes no visible problem in the year it is chosen. The form is accepted, nothing is queried, and the number appears to have done its job. The failure is deferred, and it is deferred to the point where it is most expensive to fix.

The mechanism is worth spelling out. The reference ID number is the only stable link the IRS has between one year's Form 5471 and the next for a company with no EIN. The company's name is free text and can change. The address changes. The filer's own name can change. The identifier is the anchor. When it moves without the correlation in item 1b(3), the filing history of a single UK company reads as two shorter histories of two different companies.

Earnings and profits are tracked per foreign corporation across years on Schedule J. A broken identifier chain leaves an opening balance with no corresponding closing balance in the prior year.
Previously taxed earnings and profits reported on Schedule P are inherently multi-year. Layers created in one year are distributed in a later year, and the tie between them runs through the corporation, not through the shareholder.
GILTI computations on Form 8992 pull tested income and qualified business asset investment per CFC, and the Form 8992 instructions require the reference ID number used there to match the one on Form 5471. A mismatch means a CFC on Schedule A that does not correspond to any Form 5471 in the same return.
Where a UK subsidiary or a UK limited liability partnership is a foreign disregarded entity, Form 8858 line 3c(2) carries the tax owner's reference ID number, so a change at the parent level propagates to a second form.
A change of preparer is the single most common trigger. The identifier lives in the departing preparer's software, the incoming preparer never receives it, and a new one is invented. The correlation window in item 1b(3) is the first year the new identifier is used, and by the time the omission is noticed that year has usually passed.

The consequence to be honest about is exposure rather than certainty. The penalty regime described in the Instructions for Form 5471 is severe and it is per corporation and per year: a $10,000 penalty is imposed for each annual accounting period of each foreign corporation for failure to furnish the information required by section 6038(a) within the time prescribed, with an additional $10,000 penalty per foreign corporation for each 30-day period, or fraction thereof, during which the failure continues after a 90-day period following IRS notice has expired, limited to a maximum of $50,000 for each failure. Section 6038(c) adds a reduction of 10 percent of the foreign taxes available for credit under sections 901 and 960, with an additional 5 percent reduction for each 3-month period during which the failure continues after that same 90-day period. A parallel $10,000 penalty regime applies under section 6679 for failures under section 6046 and Schedule O. None of that is triggered by a poorly chosen identifier by itself. What a broken identifier does is remove the evidence that the earlier years were filed for the same company, which is exactly the evidence a filer wants in hand if a filing history is ever questioned.

Filing several years of delinquent Forms 5471 at once

The catch-up case is where the consistency rule and the human instinct pull hardest against each other. A US citizen in London discovers a Form 5471 obligation going back several years. Six Forms 5471 are prepared over a fortnight, printed together, checked together and posted together. Everything about the process says one filing. The instructions say otherwise: each Form 5471 relates to a separate annual accounting period of the foreign corporation, and the same reference ID number must be used consistently from tax year to tax year. The fact that the years were prepared on the same afternoon changes nothing.

Two routes carry these filings, and the identification block is handled identically on both. The Delinquent International Information Return Submission Procedures at irs.gov/individuals/international-taxpayers/delinquent-international-information-return-submission-procedures, which remained live when last reviewed on 19 April 2026, are available to taxpayers who have identified the need to file delinquent international information returns, who are not under civil examination or criminal investigation and have not already been contacted by the IRS about the delinquent information returns. Under those procedures, delinquent international information returns other than Forms 3520 and 3520-A are attached to an amended income tax return and filed according to the applicable instructions for that return, and a reasonable cause statement may be attached to each delinquent information return for which reasonable cause is asserted. The page also states plainly that penalties may be assessed in accordance with existing procedures.

The Streamlined Foreign Offshore Procedures at irs.gov/individuals/international-taxpayers/u-s-taxpayers-residing-outside-the-united-states require delinquent or amended tax returns for each of the most recent 3 years for which the US tax return due date, or properly applied for extended due date, has passed, delinquent FBARs for each of the most recent 6 years for which the FBAR due date has passed, and a signed Form 14653 certifying non-residency and non-willful conduct. The IRS instructs filers to write Streamlined Foreign Offshore in red at the top of the first page of each delinquent or amended tax return and at the top of each information return. Forms 5471 for each covered year travel inside that package. Which route fits a given set of facts is a judgement made on the whole picture, and it is the first question we settle in the work described at us-uktax.com/irs-streamlined-filing.

The discipline we apply to a catch-up stack is the same regardless of route.

Establish the reference ID number before drafting the earliest year, not after finishing the latest. It is far easier to type one identifier six times than to correct five forms.
Use that identifier on every year in the package, on every schedule within each year, and on the current-year return that follows the catch-up.
Leave item 1b(3) blank in a clean catch-up. The field is headed previous reference ID number(s), if any. Where no Form 5471 was ever filed for the company, there is no previous identifier to correlate, and nothing should be invented to fill the space.
Where some earlier years were filed by a previous preparer under a different identifier, correlate once. The new identifier goes in item 1b(2) and the old identifier or identifiers go in item 1b(3), separated by a space, in the first year the new identifier is used within the package.
Apply the Form 8832 sequence at the year it belongs to, not at the date of preparation. If an entity classification election was made in the middle of the catch-up period, the first Form 5471 filed after that election is the year that carries the new EIN in item 1b(1) and the old reference ID number in item 1b(2), even though that year sits in the middle of the stack.
Confirm the same identifier on any Form 8858 and Form 8992 schedules in each year of the package before the package is assembled, not afterwards.
Keep a one-page identifier record with the engagement file recording the number, the company, its registration number and every year in which it has been used, so that the next preparer inherits it.

A closing point on catch-up work that is easy to overlook. If the delinquent years are filed under one identifier and the following year's return, prepared months later by the same firm, carries a different one, the catch-up has created the very discontinuity it was meant to cure. The identifier record is what prevents this, and it costs a single page.

The address, country and currency fields for a UK company on page 1

Item 1a on Form 5471, revision 12-2025, at irs.gov/pub/irs-pdf/f5471.pdf is a single free-text block headed name and address of foreign corporation, sitting to the left of items 1b(1), 1b(2) and 1b(3). Below it run item 1c, country under whose laws incorporated, item 1d, date of incorporation, item 1e, principal place of business, items 1f and 1g, the principal business activity code number and the principal business activity, and item 1h, the functional currency code. The form carries an instruction above the block that all information must be in English and all amounts must be stated in US dollars unless otherwise indicated.

The address conventions the Instructions for Form 5471 set out for a foreign address are to enter the information in the order city, province or state, and country, to follow the country's practice for entering the postal code if any, and not to abbreviate the country name. Applying that to a UK company means writing United Kingdom in full rather than UK or GB, and writing the postcode in the normal UK format and position rather than forcing it into a US ZIP code position. The instructions do not prescribe whether to use the registered office at Companies House or the address the company actually trades from, and the two are often different because the registered office is frequently a formation agent's address. Our practice, and it is practice rather than rule, is to use the address from which the company is actually managed where that is a real place, to keep it consistent year to year, and to record which address was used in the engagement file.

A name change deserves its own note because it interacts with the reference ID rules. The instructions state that if the name of either the person filing the return or the corporation whose activities are being reported changed within the past 3 years, the prior name or names should be shown in parentheses after the current name. That is the correct place to reflect a UK company rebrand. The reference ID number does not change, and must not change, simply because the company changed its name at Companies House. The name field carries the change; the identifier carries the continuity.

For item 1c the form asks for the country under whose laws the corporation is incorporated. For a company on the Companies House register we enter United Kingdom. The England and Wales, Scotland and Northern Ireland distinction is a matter for the registrar and is already carried by the prefix on the registration number. It is worth noting that the two-letter code the IRS uses on the Form 5471 schedules that call for a country code, from the list at IRS.gov/CountryCodes, is UK for the United Kingdom, covering England, Northern Ireland, Scotland and Wales as a single entry, with no separate codes for the constituent countries.

Items 1f and 1g catch people out. The instructions direct the filer to enter the principal business activity code number and the description of the activity from the list at the end of the Form 5471 instructions. That is a US activity code list. The SIC code shown against the company on the Companies House record is a UK classification and is not the code the IRS is asking for. The two rarely correspond neatly, and the correct approach is to work from the US list and document why the chosen code fits.

Item 1h asks for the functional currency code. The instructions state that the foreign corporation's functional currency is determined under section 985, and that the filer should enter the applicable three-character alphabet code using the ISO 4217 standard. For a UK company whose functional currency is sterling that is GBP. Functional currency is a determination, not an assumption, and for a UK company that invoices predominantly in dollars or euros it needs to be reasoned through rather than defaulted. The currency choice then drives the translation work across Schedule C, Schedule F and Schedule H, and the instructions require exchange rates to be reported using a divide-by convention rounded to at least four places, expressed as the units of foreign currency that equal one US dollar.

A worked example: a three-company UK group across a decade of filings

The following is an illustration rather than a client file, and the company names and numbers are invented. Assume a US citizen resident in London who owns the whole of Hartwell Holdings Ltd, registered in England and Wales under number 09876543. Hartwell Holdings owns Hartwell Trading Ltd, registered under number 10233445, and Hartwell Energy Ltd, registered in Scotland under number SC456789. None of the three has ever held a US employer identification number. Forms 5471 are required for all three.

At the outset, three reference ID numbers are established and recorded: UK09876543 for the holding company, UK10233445 for the trading company and UKSC456789 for the Scottish company. Each is alphanumeric, carries no special characters or spaces, is well inside the 50-character limit, and is unique. Nothing is filed with the IRS to establish them, because nothing needs to be. They are entered in item 1b(2) of the respective Forms 5471, item 1b(1) is left empty because there is no EIN, and item 1b(3) is left empty because there is no previous identifier.

In year four, Hartwell Trading Ltd is renamed Northgate Systems Ltd. The registration number does not change, so the reference ID number does not change either. Item 1a carries Northgate Systems Ltd with Hartwell Trading Ltd in parentheses after it for the three years following the change, and item 1b(2) continues to carry UK10233445 in every year without interruption. This is the single most valuable property of a registration-number-derived scheme: the event that most often prompts an inexperienced preparer to invent a new identifier has no effect on it at all.

In year six an entity classification election is made on Form 8832 on behalf of Hartwell Energy Ltd, and an EIN is obtained for the company because Regulations section 301.6109-1(b)(2)(v) requires one for that election. On the first Form 5471 filed after the election, the new EIN goes in item 1b(1) and UKSC456789 goes in item 1b(2), exactly as the instructions direct. In year seven and afterwards the EIN remains in item 1b(1), which the instructions require, and UKSC456789 stays in item 1b(2), which the instructions permit. Item 1b(3) is never used for this company, because the reference ID number was never replaced by a different reference ID number.

Also from year six, the group holds a UK limited liability partnership treated as a foreign disregarded entity with Hartwell Holdings as its tax owner. Form 8858 line 3c(2) carries UK09876543, matching item 1b(2) on the holding company's Form 5471. Where Schedule A (Form 8992) is filed for the CFCs, column (b) carries the same identifiers, matching the Forms 5471 for the same year, as both sets of instructions require.

In year nine, Northgate Systems Ltd is sold. Its final Form 5471 is filed, the final Form 5471 box on page 1 is checked, and UK10233445 is marked retired in the identifier record. It is never used again for any other company, because the instructions prohibit reusing a reference ID number that has fallen out of use. When the group incorporates a replacement company in year ten under a new registration number, that company receives its own identifier derived from its own registration number. No decision is required, because the convention makes the answer automatic.

Across ten years, three companies, one rename, one entity classification election, one disposal, one incorporation and, in the real version of this story, at least one change of accountant, not one identifier moved. That is the entire objective.

What we check when we inherit a file with inconsistent reference ID numbers

Where prior-year Forms 5471 exist and the identification block is inconsistent, the position has to be reconstructed before the current year is prepared, not after. The sequence we work through is deliberately unglamorous.

Obtain every prior-year Form 5471 actually filed, from the client's copies and, where those are incomplete, from IRS transcripts and records of the returns to which they were attached.
List, for each company and each year, exactly what appeared in items 1b(1), 1b(2) and 1b(3), including any placeholder strings.
Establish whether any EIN exists for each company, and whether an entity classification election was ever made on Form 8832.
Confirm each company's registration number and incorporation date against the public register at find-and-update.company-information.service.gov.uk, and note every historic name.
Decide whether to adopt the most recently used compliant identifier or to establish a new one. The default is to adopt the existing one, because continuity beats tidiness and no correlation entry is then required.
Where a new identifier is unavoidable, correlate it once in item 1b(3) in the first year the new identifier is used, separating multiple prior identifiers with a space.
Check the same identifier through every Form 8858 line 3c(2) and every Schedule A or Schedule B (Form 8992) in the same return.
Record the outcome in a one-page identifier register held with the permanent file, and hand it over on any future change of preparer.

None of this is difficult work. It is simply work that has to be done once, early, by someone who understands that the number in item 1b(2) is not an administrative formality but the spine of a filing history. Form 5471 preparation for UK company owners sits inside our wider US and UK compliance work at us-uktax.com/us-tax-services and us-uktax.com/cross-border-tax-planning, and for owners building or restructuring a UK group with US shareholders, the identification decisions belong at the structuring stage described at us-uktax.com/us-uk-business-expansion rather than at the filing deadline. If you are carrying several unfiled years for a UK company, the identifier is one of the first things we fix, and you can start that conversation at us-uktax.com/contact.