Do US wash sale rules apply to UK shares held with a UK broker?

Yes. Section 1091 applies to any sale of stock or securities at a loss by a US taxpayer, with no exception for foreign securities, foreign brokers or foreign currency. A US citizen selling a UK-listed share through a London platform is inside the rule. The practical difference is that no UK broker issues a Form 1099-B or computes a wash sale, so the entire analysis and the resulting basis adjustment have to be built by hand from contract notes.

Is the UK bed and breakfast rule the same as the US wash sale rule?

No. They target the same behaviour with different machinery. Section 106A TCGA 1992 does not disallow anything: it matches the disposal with acquisitions in the 30 days following, so the gain or loss is recomputed against the repurchase cost instead of the section 104 pool. Section 1091 disallows the loss outright and adds it to the basis of the replacement shares. The UK rule also looks only forward, while the US rule looks 30 days back as well.

Does a bed and ISA transaction trigger a US wash sale?

Yes. Bed and ISA defeats the UK rule because section 106A requires an acquisition by the same person in the same capacity, and an ISA purchase is treated differently for UK matching. Section 1091 has no such requirement: it is the same taxpayer buying substantially identical stock inside the 61-day window, so the US loss is disallowed. The disallowed amount rolls into the basis of the ISA holding, which a US person must then track, since an ISA gives no US tax shelter.

Which tax year does the disallowed loss fall in when the UK year ends 5 April?

The disallowance always falls in the US tax year of the sale, which ends on 31 December. That is why an early April sale with a post 5 April repurchase produces a UK loss in one tax year and no US loss at all in a differently dated year, and why a December sale with a January repurchase gives a US disallowance on the earlier calendar year return while the UK treats both legs as one tax year to 5 April.

Can dividend reinvestment on a UK platform create a wash sale?

Yes, where the reinvestment actually buys new units or shares. Section 1091 requires an acquisition, not an intention, so an automatic reinvestment, a model portfolio rebalance or a monthly regular investment inside the 61-day window is a replacement purchase. Accumulation units are different: HMRC guidance confirms that no new units are issued and instead the value of the existing holding is increased, which makes an accumulation reinvestment a much weaker candidate for a section 1091 acquisition.

Are ADRs substantially identical to the underlying UK ordinary shares?

The conservative and generally accepted position is yes, because an American Depositary Receipt is a receipt over the same underlying UK ordinary share and carries essentially the same economic rights. Publication 550 asks taxpayers to weigh all the facts and circumstances rather than applying a bright-line test, and states that securities of one corporation are ordinarily not substantially identical to those of another. That helps with sector substitution, but it does not help where the ADR and the ordinary represent the same company.

US Wash Sale Rules on UK Shares for US Investors

The US wash sale rules UK shares investors run into are set by section 1091 of the Internal Revenue Code, and they are blunt: if you sell stock or securities at a loss and, within a period beginning 30 days before the date of that sale and ending 30 days after it, you acquire or contract to acquire substantially identical stock or securities, no deduction is allowed for the loss. The statutory language is at https://www.law.cornell.edu/uscode/text/26/1091. Nothing in that section carves out foreign shares, foreign brokers or foreign currency. A US citizen selling a FTSE 100 holding through a London platform is inside section 1091 exactly as if the trade had happened on the New York Stock Exchange.

The complication is that the United Kingdom polices the same behaviour with a completely different instrument. HMRC does not disallow anything. It re-matches the disposal, under the same day rule in section 105(1) TCGA 1992 and the 30 day rule in section 106A TCGA 1992, so that the shares you sold are treated as the shares you just bought back. The gain or loss is then recomputed against the repurchase cost instead of against the pooled cost in your section 104 holding. Two systems, two mechanisms, two timelines, and a lookback window that exists in one country and not the other.

In the returns we prepare for US persons holding UK-listed equities and UK-domiciled funds, this is one of the most reliable sources of an amended return. The failure mode we see most often is not aggressive tax-loss harvesting. It is an investor who did something completely ordinary on a UK platform, in a UK tax year, on UK advice, and then discovered that the US side of the file had a disallowed loss sitting in it that nobody had tracked.

What are the US wash sale rules UK shares investors face?

A wash sale is a sale of stock or securities at a loss where substantially identical stock or securities are acquired within the 61-day period made up of the 30 days before the sale, the day of the sale itself, and the 30 days after it. Section 1091(a) denies the deduction that would otherwise be available under section 165 unless the taxpayer is a dealer in stock or securities and the loss arose in the ordinary course of that business. That dealer exception is narrow and does not cover an investment banker trading a personal account.

The word acquired is broader than buy. Section 1091(a) captures a purchase, an exchange on which the entire amount of gain or loss was recognised, and entering into a contract or option to acquire. Section 1091(f) confirms the section still applies to a contract or option that settles in cash rather than in the shares themselves. Section 1091(e) extends similar rules to a loss realised on closing a short sale or terminating a securities futures contract to sell. The implementing regulation at https://www.law.cornell.edu/cfr/text/26/1.1091-1 sets out how acquisitions are matched to sales in order of acquisition, earliest first, when the numbers on each side do not line up.

Three features of the rule cause most of the cross-border damage. First, it looks backwards as well as forwards, so a purchase you made three weeks before you decided to sell can disallow the loss on the sale. Second, it is a taxpayer-level rule, not an account-level rule, so a repurchase in a different account, at a different broker, in a different country still counts. Third, it operates in US dollars, because IRS guidance at https://www.irs.gov/individuals/international-taxpayers/foreign-currency-and-currency-exchange-rates requires amounts on a US return to be expressed in US dollars using the exchange rate prevailing when you receive, pay or accrue the item.

How the disallowed loss lands in the basis of your replacement shares

A wash sale defers a loss, it does not delete it. Section 1091(d) provides that the basis of the replacement property is the basis of the stock or securities sold, increased or decreased by the difference between the price at which the replacement was acquired and the price at which the sold shares went out. Publication 550 at https://www.irs.gov/publications/p550 puts the same rule in practical terms: add the disallowed loss to the cost of the new stock, and that is your basis in the new stock. The economic loss is preserved and surfaces when you eventually sell the replacement shares in a transaction that is not itself a wash sale.

The second half of the mechanic is the holding period. Publication 550 confirms that the holding period of the shares you sold is added to the holding period of the replacement shares. That is genuinely useful for a US investor rotating in and out of a UK position, because it protects long-term capital gain treatment on the replacement lot even though the replacement was bought days ago. It is also a trap in reverse: if the disallowed sale was of a short-term lot, that short holding period tacks on and does not magically become long-term.

Where the deferral breaks down is when the replacement purchase happens inside a wrapper whose basis cannot be adjusted. Revenue Ruling 2008-5, published at https://www.irs.gov/pub/irs-drop/rr-08-05.pdf, holds that where an individual sells stock at a loss and causes their IRA to acquire substantially identical stock inside the 61-day window, the loss is disallowed under section 1091 and the individual's basis in the IRA is not increased. The loss is simply gone. A UK Individual Savings Account is a different animal, because for US purposes it is an ordinary taxable account owned by the same individual, so a repurchase inside an ISA does get the section 1091(d) basis uplift. That is a small mercy, and it is the only comfort an ISA offers a US person.

The disallowed amount is added to the cost of the replacement shares under section 1091(d), so the loss is deferred rather than lost.
The holding period of the shares you sold is added to the holding period of the replacement shares.
Where fewer replacement shares are acquired than were sold, only a proportionate part of the loss is disallowed, matched under Reg 1.1091-1 in order of acquisition beginning with the earliest.
A repurchase inside a retirement arrangement can destroy the loss permanently rather than defer it, per Revenue Ruling 2008-5.
The rule bites on losses only. Section 1091 has nothing to say about a gain, which is why a UK bed and breakfast trade done to use the annual exempt amount raises different questions on each side.

What does substantially identical mean for UK share classes, ADRs and funds?

Congress never defined substantially identical, and the IRS has never issued a bright-line test. Publication 550 tells taxpayers to weigh all the facts and circumstances, and states the general position that securities of one corporation are ordinarily not substantially identical to securities of another corporation, while flagging that bonds or preferred stock convertible into the common stock of the same corporation may be substantially identical to that common stock where the terms make them effectively interchangeable. Everything else is judgement, and for cross-border portfolios the judgement calls come thick and fast.

The commonest live questions we field in US-UK portfolios are these. An American Depositary Receipt over a UK ordinary share is, in substance, a receipt for that same ordinary share, so treating an ADR and the underlying UK ordinary as substantially identical is the conservative and generally accepted position. Two share lines in the same UK issuer created by a historic dual-listed structure require a hard look at whether the economic rights really are interchangeable. A UK OEIC and a UK-listed exchange traded fund tracking the identical index, with the same constituents and weightings, sit uncomfortably close to identical even though they are different legal entities. An OEIC and an actively managed fund with a different mandate in the same sector are not.

ADRs over a UK ordinary share and the ordinary share itself should be treated as substantially identical.
Income units and accumulation units of the same UK fund are units in the same fund, differing only in distribution policy, so they should be treated as substantially identical.
Two index funds from different managers tracking the same index with the same methodology are a high-risk pairing, not a safe harbour.
A UK ordinary share and a call option over that share are caught, because section 1091(a) expressly reaches a contract or option to acquire.
Ordinary shares of two different UK companies in the same sector are ordinarily not substantially identical, which is why sector substitution remains the standard way to stay invested through a harvest.
GBP-hedged and unhedged share classes of the same fund are the same underlying fund, and the hedge alone is unlikely to make them distinguishable.

How does HMRC match the same trade? Same day, 30 days and the section 104 pool

The UK approach is identification, not disallowance. HMRC helpsheet HS284 at https://www.gov.uk/government/publications/shares-and-capital-gains-tax-hs284-self-assessment-helpsheet/hs284-shares-and-capital-gains-tax-2024 sets the order. A disposal is matched first with shares acquired on the same day as the disposal, then with shares acquired in the 30 days following the day of disposal provided the person making the disposal was resident in the UK at the time of that acquisition, and only then with the section 104 holding. Anything still unmatched goes against later acquisitions, earliest first. The Capital Gains Manual at https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg51560 confirms the statutory hooks, section 105(1) TCGA 1992 for the same day rule and section 106A(5) and (5A) TCGA 1992 for the 30 day rule, and states that the 30 day rule has priority over all other identification rules except the same day rule.

The section 104 holding is the default. HMRC guidance at https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg51575 describes all the shares in a section 104 holding as indistinguishable parts of a single asset, made up of a single pool of expenditure, apportioned on a partial disposal by reference to the number of shares sold. In other words, the UK works with an averaged pool cost, while the US works with lot-by-lot basis. Those two accounting models never produce the same number on a partial disposal, and that divergence exists before you add a wash sale to it.

Three structural differences matter for planning. The UK 30 day rule looks only forward, at acquisitions in the 30 days after the disposal, whereas section 1091 also looks back 30 days. The UK rule requires shares of the same class in the same company acquired by the same person in the same capacity, which is a far narrower net than substantially identical. And the UK rule switches off entirely where the acquisition happens at a time the individual is not UK resident. HMRC states that point explicitly for relevant securities at https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg51565, where acquisitions made while the taxpayer is non-resident or treaty non-resident are ignored for the 30 day matching rule.

Why a clean UK bed and breakfast trade is still a US wash sale

HMRC describes bed and breakfasting at https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg13350 as arrangements in which a person sells an asset only to buy it back again a short time later. The classic UK responses to the 30 day rule are bed and ISA, where the shares are sold in a general investment account and repurchased inside an Individual Savings Account, and bed and spouse, where the shares are repurchased by a spouse or civil partner in their own name. Both work in the UK because they defeat the same person in the same capacity requirement in section 106A.

Neither works for the US. A bed and ISA is the same taxpayer buying substantially identical stock inside the 61-day window, so section 1091 disallows the loss in full, and the disallowed amount rolls into the basis of the ISA holding, which the investor now has to track for the rest of their life on a wrapper the US does not recognise as tax-free anyway. Bed and spouse is worse in a different way, because the IRS position is that a purchase by a spouse inside the window is a wash sale of the seller, and where a US couple file jointly the economic unit is transparently the same. A UK adviser who executes either trade for a US citizen has solved a UK problem and created a US one.

The mirror image also happens. A US person who has left the UK, or who is treaty non-resident, can repurchase within 30 days with no UK matching consequence at all because the UK rule ignores acquisitions made while non-resident, and can still be squarely inside section 1091 on the US return. The two rules genuinely do disengage from one another, and the only way to know which of them is live is to run both.

Gap angle one: the 5 April versus 31 December year-end trap

The UK tax year runs 6 April to 5 April, as confirmed on the GOV.UK capital gains pages at https://www.gov.uk/capital-gains-tax/rates. The US individual tax year is the calendar year ending 31 December. The 61-day wash sale window does not care about either boundary, and that produces two distinct mismatches that competitor pages simply do not address.

The first is the April problem. A UK investor selling on 2 April to bank a loss before the UK year end, and repurchasing on 8 April to sit outside the section 106A window running forward from the disposal, has done something that looks disciplined. The repurchase falls in the new UK tax year, so the UK computation is untouched. But 2 April and 8 April are six days apart in the same US calendar year, so the entire US loss is disallowed under section 1091, and the disallowed amount attaches to the April lot. The investor gets a UK loss for the year ended 5 April and no US loss at all for the year ended 31 December. If those are different rate contexts, and for a high-net-worth investor they usually are, the foreign tax credit and the loss relief now sit in different places and cannot be matched against each other.

The second is the January problem. A December sale at a loss followed by a January repurchase inside 30 days is a wash sale for the US year that just ended, so the loss is disallowed on the return for the earlier calendar year even though the triggering purchase happened in the later one. Both dates sit inside the same UK tax year to 5 April, so on the UK side the disposal is simply matched with the January repurchase under section 106A and the gain or loss is recomputed against that cost. One economic decision, one UK tax year, two US tax years, and a disallowance that has to be reported on the earlier US return. In the returns we prepare, this is the single most common reason a December harvest never actually reduces a US tax bill.

Sale in early April with a repurchase after 5 April: no UK matching consequence, full US disallowance, loss deferred into the new lot.
Sale in December with a repurchase in January: US disallowance falls in the earlier calendar year, UK matching falls in the same UK tax year, and the two returns tell different stories.
Sale in February with a repurchase in March: both systems bite, the US disallows and the UK re-matches, and the UK figure and the US figure will still differ because the UK used pooled cost and the US used lot basis.
Any sale where a purchase was already made in the preceding 30 days: the US disallows on the lookback, the UK is indifferent because section 106A only looks forward.

Gap angle two: automatic reinvestment inside a UK platform buys shares you never chose

Section 1091 does not require intent. It requires an acquisition. UK investment platforms are full of standing instructions that generate acquisitions the investor never consciously made, and each of them can silently disallow a loss. Automatic dividend reinvestment on income units buys new units, often on an ex-dividend cycle the investor does not track. Model portfolio rebalancing buys back a position that was trimmed weeks earlier. A monthly regular investment instruction keeps buying through a harvest. A drip-feed from a cash account into a target allocation does the same. Every one of those is a purchase of the same security by the same taxpayer, and if it lands inside the 61-day window it does exactly what a deliberate repurchase would do.

Accumulation units behave differently, and the distinction is worth knowing precisely. HMRC guidance at https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg57707 states that with accumulation units no new units are issued and instead the value of the existing holding of units is increased, with the notional distribution treated as allowable expenditure where it has been subject to Income Tax. Because no new units come into existence, an accumulation reinvestment is a much weaker candidate for a section 1091 acquisition than a distribution reinvestment that actually buys units. That is a helpful structural point when choosing which share class to hold while harvesting, and it is one no US-focused wash sale page mentions because accumulation units barely exist in the US market.

The reporting consequence is where this really bites. A UK broker does not issue a Form 1099-B, does not compute a wash sale and does not report anything to the IRS. A US broker does report wash sales, but only within its own accounts and generally only for identical securities. An investor holding the same exposure through a London platform and a US account has no automated safety net at all. The reconciliation has to be built by hand from contract notes, and it has to include every automatic purchase on both sides of the Atlantic.

Switch off dividend reinvestment on any holding you intend to harvest, at least 31 days before the planned disposal and for 31 days after it.
Suspend model portfolio rebalancing and regular monthly investment instructions across the whole 61-day window, not just after the sale.
Check the ex-dividend and payment dates on UK income units, because the reinvestment purchase settles on the payment date, not the date you thought about it.
Remember that a purchase in a spouse's account, a jointly controlled account or a wrapper you fund can all be replacement purchases.
Do not rely on a UK consolidated tax certificate to find any of this. It is built for Self Assessment and reports nothing a section 1091 analysis needs.

PFIC and Form 8621: the replacement purchase starts a fresh problem

For a US person, a UK-domiciled OEIC, unit trust or investment company is generally a passive foreign investment company. Form 8621 is filed by a direct or indirect US shareholder of a PFIC who receives certain distributions, recognises gain on a disposition, makes or reports a qualified electing fund or section 1296 mark-to-market election, or reports under section 1298(f). The IRS overview is at https://www.irs.gov/forms-pubs/about-form-8621.

That matters here for three reasons. A disposal of a PFIC holding is a reportable event in its own right, so a harvest of a UK fund position pulls a Form 8621 into the return whether or not the loss survives. A replacement purchase creates a new PFIC holding, with its own holding period for excess distribution purposes if no election is in place. And the interaction between the section 1091 basis adjustment and PFIC computations is technical enough that it should never be left to a broker report or a software default. Where a section 1296 mark-to-market election is in force, the annual mark itself changes the character of what is being recognised, and a wash sale analysis has to be run against the correct measure rather than against the raw sale proceeds. We treat any UK fund harvest for a US person as a Form 8621 exercise first and a wash sale exercise second.

A worked illustration: one UK disposal, two tax systems

The following figures are illustrative only and use an assumed exchange rate of 1.30 US dollars to the pound on every relevant date, purely so the arithmetic is readable. Real computations must use the actual rate prevailing on each date under the IRS guidance cited above, and a moving rate can change the answer entirely.

Assume a US citizen resident in London holds 10,000 shares in a FTSE 100 industrial company. The section 104 pool cost is 80,000 pounds, an average of 8.00 pounds per share. For US purposes the shares were bought in two lots, 6,000 at 9.00 pounds and 4,000 at 6.50 pounds. On 20 March the investor sells all 10,000 shares at 6.00 pounds, receiving 60,000 pounds. On 10 April, three weeks later, the investor buys 10,000 shares back at 6.20 pounds, in the new UK tax year.

On the UK side the 10 April repurchase falls within the 30 days following the 20 March disposal, so section 106A matches the disposal to the repurchase rather than to the pool. The disposal proceeds of 60,000 pounds are set against the 62,000 pound repurchase cost, producing an allowable loss of 2,000 pounds, and the section 104 pool of 80,000 pounds survives untouched, still attached to the 10,000 shares the investor now holds. The 78,000 pound economic loss the investor thought they had crystallised has not been crystallised at all. It is still locked in the pool.

On the US side the analysis is different in every respect. Translated at the assumed rate, the sale realises 78,000 US dollars of proceeds against a lot basis of 70,200 dollars on the 9.00 pound lot and 33,800 dollars on the 6.50 pound lot, so there is a realised loss in dollars. Because substantially identical stock was acquired on 10 April, well inside the 30 days after the sale, section 1091(a) disallows the entire loss. Under section 1091(d) the disallowed amount is added to the cost of the April purchase, so the new US basis is materially higher than the 80,600 dollars actually paid, and the holding period of both original lots tacks onto the replacement shares. The investor files Form 8949 showing the sale with code W and the disallowed loss as a positive number in column (g), and carries a US basis figure that no statement anywhere in the world will ever show them.

How do you report a UK share wash sale on Form 8949 and Schedule D?

The mechanics are set out in the Form 8949 instructions at https://www.irs.gov/instructions/i8949. Report the sale on Form 8949, enter W in column (f) as the adjustment code, and enter the amount of the nondeductible loss as a positive number in column (g). The instructions are explicit that if a broker statement shows an incorrect wash sale amount, you enter the correct amount as a positive number in column (g) rather than the reported one, which is precisely the situation for anyone whose replacement purchase happened on a UK platform that reports nothing.

The totals flow to Schedule D. The Schedule D instructions at https://www.irs.gov/instructions/i1040sd confirm that capital losses are deductible against capital gains plus 3,000 US dollars, or 1,500 dollars if married filing separately, with any excess carried forward. That interaction is the reason a disallowed wash sale loss is more expensive than it looks for a high earner: the loss you were counting on to absorb a large gain elsewhere in the portfolio is simply not there, and the shortfall does not fall back on the ordinary income deduction in any meaningful size.

Every UK share transaction goes on Form 8949 in US dollars, translated at the rate prevailing on the trade or settlement date used consistently.
Wash sale adjustment code W goes in column (f), and the disallowed loss goes in column (g) as a positive number.
UK transactions with no Form 1099-B are reported in the Form 8949 category for transactions not reported to the IRS on a Form 1099-B.
Keep a standing basis schedule for each UK holding showing lot dates, sterling cost, translated dollar cost, and every section 1091(d) adjustment ever applied.
Reconcile that schedule against the UK section 104 pool every year, because the two numbers are supposed to differ and you need to be able to explain why.

Currency: there has to be a dollar loss before section 1091 can bite

This is the point most cross-border investors miss entirely. Section 1091 applies to a loss, and for US purposes the loss is measured in dollars. IRS guidance requires amounts on a US return to be expressed in US dollars using the exchange rate prevailing when the item is received, paid or accrued. So the sterling proceeds are translated at the rate on the disposal date and the sterling cost of each lot is translated at the rate on that lot's acquisition date.

The consequence is that a UK share can fall in sterling terms over a period in which the pound strengthens against the dollar, leaving a sterling loss that is a dollar gain. When that happens there is no US loss, section 1091 is irrelevant, and the whole wash sale analysis falls away, even though HMRC will still re-match the disposal under section 106A if a repurchase lands inside 30 days. The reverse also happens, and more painfully: a sterling gain can be a dollar loss, in which case an investor who did not think they had harvested anything can have a US loss disallowed by an unrelated repurchase. Currency is not a rounding adjustment in this analysis. It determines whether the rule engages at all.

The checks we run before signing off a cross-border share disposal

Pull every purchase of the security, in every account and every wrapper controlled by the client or their spouse, for the 30 days before and the 30 days after the disposal date.
Confirm whether any standing instruction, dividend reinvestment, rebalance or regular saver executed inside that window.
Translate proceeds and each lot basis into dollars at the correct dated rates and confirm there is a dollar loss before going further.
Test every candidate replacement security against substantially identical, treating ADRs, unit classes of the same fund and same-index trackers as high risk.
Run the UK matching separately under the same day rule, then section 106A, then the section 104 pool, and record why the UK and US numbers differ.
Check whether the disposal or the replacement is a PFIC interest requiring Form 8621, and identify any section 1296 or QEF election already in place.
Carry forward a written basis schedule with every section 1091(d) adjustment, because nobody else in the chain is keeping it.

The underlying point is simple even if the execution is not. Section 1091 and section 106A TCGA 1992 were written to stop the same behaviour, but they were written independently, they use different definitions, they run on different timelines, and they belong to tax years that do not line up. A trade that is entirely clean under one is routinely blocked by the other. For a US person investing through UK brokers and UK-domiciled funds, the only reliable approach is to run both computations on every loss-making disposal, in both currencies, before the trade is placed rather than in the following spring when the paperwork arrives.