Introduction: Why Money Market Funds Are Not Cash for Americans in Britain
British money market funds feel like a bank account with a better rate, yet the IRS treats each one as a share in a foreign investment company. That single difference changes the tax, the forms and the penalties. Consequently, wealthy Americans in Britain often hold their safest asset in their most awkward wrapper.
Moreover, many holders never chose the fund. A private bank, a discretionary manager or an investment platform swept idle cash into it automatically. The statement shows a stable balance and a monthly income line. Nothing on it warns a US citizen that a separate American form may be due for that holding every year.
At TaxYork, we prepare US and UK returns for Americans with substantial sterling cash. Therefore, this guide explains how both countries tax money market funds, where the two systems diverge, and how to correct missed reporting. It also includes a worked example with real figures.
How Money Market Funds Work in Britain
Sterling money market funds are collective investment schemes that hold short-dated deposits, commercial paper and government bills. Most target a constant price of £1 a share and pay out income monthly. Others accumulate income, so the share price climbs steadily instead. Importantly, many funds sold in Britain are domiciled in Ireland or Luxembourg rather than the UK.
Their yield tracks short-term interest rates, which follow the Bank of England's Bank Rate. For that reason, wealthy investors use money market funds to park sale proceeds, bonus payments and tax reserves. A balance of several million pounds is common after a business exit or a property sale.
Who This Guide Is For
This guide is for US citizens and green card holders living in Britain who hold significant sterling cash. It applies whether you bought a fund deliberately or a manager placed you in one. Furthermore, it applies to British spouses who hold joint accounts with an American, because the American's share is still reportable.
How Britain Taxes Money Market Funds
Britain taxes the income from money market funds as savings interest, not as dividends. A fund that holds more than 60% of its assets in interest-bearing investments pays interest distributions. For offshore funds, section 378A of the Income Tax (Trading and Other Income) Act 2005 produces the same result. Therefore, the label "dividend" on a statement does not decide the UK treatment.
Rates and Allowances for 2026/27
For 2026/27, savings income is taxed at 20%, 40% and 45%, as the HMRC income tax rate tables show. Additionally, the personal savings allowance covers £1,000 of interest for basic-rate taxpayers and £500 for higher-rate taxpayers. However, additional-rate taxpayers receive no allowance at all.
Furthermore, the government has announced that savings rates will rise by two points from 6 April 2027. The top rate on interest will then reach 47%. Consequently, a high earner with £2 million in a cash fund yielding 4% faces UK tax of £36,000 this year and £37,600 once the new rates apply.
Accumulation Shares and Offshore Funds
Accumulation shares pay nothing out, yet Britain still taxes the income each year. For a UK fund, the reinvested amount is taxable as it arises. For an offshore fund, the answer depends on reporting fund status. A reporting fund passes its income to you annually as excess reportable income. In contrast, a non-reporting fund produces an offshore income gain on sale, as HMRC's investment funds manual explains.
You can check a fund against HMRC's list of approved offshore reporting funds. Our guide to reporting fund status for US investors covers the detail. Notably, a cash fund's gain is all income in any case, so the UK rate barely changes. The timing changes, however, and timing drives the US credit.
No Capital Gain in Sterling Terms
A constant-price fund produces no UK capital gain, because you buy and sell at £1. Britain therefore taxes only the income. Meanwhile, America measures everything in dollars. As a result, a holding that never moves in sterling can still produce a large American gain or loss.
How the IRS Sees Money Market Funds Held Outside America
The IRS treats non-US money market funds as passive foreign investment companies. A foreign corporation meets the definition in section 1297 of the Internal Revenue Code if 75% of its income is passive or 50% of its assets produce passive income. A cash fund earns nothing except interest. Therefore, it meets both tests every year without exception.
The Default Regime and Its Interest Charge
Unless you elect otherwise, the default rules in section 1291 apply. Ordinary distributions are taxed as ordinary income in the year you receive them. However, an excess distribution is spread back over your whole holding period. The part allocated to earlier years is then taxed at the highest rate for each year, with an interest charge on top.
Additionally, the whole gain on a sale or redemption counts as an excess distribution. No long-term capital gain rate ever applies. Losses, in contrast, remain capital losses. Consequently, the regime taxes the upside as heavily as possible and relieves the downside as slowly as possible.
Why Rising Interest Rates Create Excess Distributions
An excess distribution is the part of a year's distributions that exceeds 125% of the average for the three preceding years. The Form 8621 instructions set out the calculation. For most funds, steady income never breaches the limit. Sterling money market funds are different, because their income moves directly with interest rates.
For example, suppose you held £1 million from 2020. Assume the fund paid £2,000 in 2020, £500 in 2021 and £14,000 in 2022. The average is £5,500, and 125% of that is £6,875. If the fund then paid £46,000 in 2023, about £39,125 became an excess distribution. Hence, ordinary interest income was thrown back to 2020 and charged at top rates with interest.
Notably, distributions in the first tax year you hold the shares can never be excess distributions. The problem therefore arises from the second year onwards. Many Americans who held sterling money market funds through the rate rises of 2022 and 2023 have this exposure and do not know it.
The Currency Gain Hiding in a £1 Share
America computes your gain in dollars. Suppose you buy £2 million of shares when the pound stands at $1.26 and redeem when it stands at $1.34. Your sterling gain is nil. Your dollar gain, however, is $160,000. Under the default regime, that entire amount is an excess distribution on redemption.
Compare a bank deposit. A currency gain on a sterling deposit is also ordinary income, as our guide to foreign currency gains on GBP accounts explains. However, a deposit carries no throwback and no interest charge. Furthermore, a currency loss on a deposit is an ordinary loss, whereas a loss on fund shares is a capital loss. The fund therefore produces a worse answer in both directions.
Form 8621, FBAR and Form 8938 for Money Market Funds
Holding money market funds outside America can trigger three separate reports. Each has its own threshold and its own consequence for omission. In our experience, clients have usually filed one or two and missed the third.
When Form 8621 Is Required
Form 8621 is filed separately for each fund, with your tax return. An exception under Treasury Regulation 1.1298-1 removes the annual filing where all your default-regime funds together are worth $25,000 or less at the year end. The limit is $50,000 on a joint return. However, the exception fails in any year with an excess distribution or a taxable gain.
Importantly, the exception removes the form, not the tax regime. Moreover, wealthy holders exceed the limit many times over. A further exception covers funds held inside a pension that the US-UK treaty protects. An ISA, however, receives no such protection, so a cash fund inside an ISA still needs the form.
FBAR and Form 8938
An account that holds fund shares is a foreign financial account for FBAR purposes. A direct holding in a fund that issues shares to the public is reportable as well. Therefore, you report the maximum value under the FinCEN foreign account rules once your combined foreign accounts exceed $10,000. Non-wilful penalties currently run to $16,536 per report.
Form 8938 applies as well. For a single American abroad, the threshold is $200,000 at the year end or $300,000 at any time. Joint filers abroad have $400,000 and $600,000. A fund already reported on Form 8621 need only be counted and cross-referenced. Our FBAR and FATCA reporting service keeps the three forms consistent.
What Missed Reporting on Money Market Funds Costs
Form 8621 carries no fixed penalty of its own. Nevertheless, its absence is serious. Under section 6501 of the Internal Revenue Code, the assessment period for the return can stay open until three years after you supply the missing form. Consequently, the IRS can revisit a year that would otherwise have closed long ago.
Additionally, a missing Form 8938 carries a $10,000 penalty, and unreported income attracts accuracy penalties and interest. Missed reporting on an investment account therefore compounds quietly. The longer the cash sits in the fund, the longer the throwback period becomes.
Elections That Soften the Rules on Money Market Funds
Two elections can replace the default regime for money market funds. Both must normally start in the first year you hold the shares. Therefore, the decision belongs at the point of purchase, not at the point of sale.
The Mark-to-Market Election
The mark-to-market election in section 1296 taxes the annual change in dollar value as ordinary income. It removes the throwback and the interest charge. Fund shares qualify if they meet the conditions in Treasury Regulation 1.1296-2. Broadly, the fund must be widely held, regulated, and redeemable at a published net asset value.
However, the election has limits. Losses are deductible only up to gains you previously included. Furthermore, a currency-driven rise in a year you sell nothing still creates taxable income, with no UK tax to credit against it. An election made late also triggers the default rules on the first year's gain.
The Qualified Electing Fund Election
A qualified electing fund election taxes your share of the fund's income each year, in the fund's own character. It needs an annual information statement from the fund. In practice, very few funds marketed to British investors supply one. Therefore, this election is rarely available, although it is worth asking the manager before you invest.
Why the Elections Rarely Fix the Past
Neither election repairs earlier years by itself. If you have held a fund for several years without one, the default regime taints the holding. You must then either accept default treatment on exit or make a purging election that triggers the charge now. Accordingly, the cleanest answer is often to leave the fund and hold the cash differently.
Credits, the Investment Income Tax and Alternatives to Money Market Funds
The UK tax on income from money market funds usually exceeds the US income tax on the same income. A 45% UK charge more than covers a 37% US charge through the foreign tax credit. However, three gaps remain, and each one costs money.
Where the Foreign Tax Credit Falls Short
First, the 3.8% net investment income tax applies to fund income and gains. On 31 August 2026, a US appeals court held that the treaty gives no credit against that tax. Second, the interest charge on an excess distribution is interest, not tax, so no credit reduces it. Third, a dollar gain caused by currency has no UK tax attached at all.
Furthermore, timing can strand credits. An offshore accumulation fund without reporting status is taxed in Britain only on sale. America, in contrast, taxes a mark-to-market holder every year. As a result, the US tax arrives years before the UK tax that should offset it.
Why American Money Market Funds Are Not the Answer
American money market funds avoid the foreign fund rules, yet they create two fresh problems. First, UK and European product rules stop most firms from selling US funds to British retail clients, as our guide to US ETFs for Americans in Britain explains. Second, HMRC taxes the income as interest at up to 45% and measures everything in sterling.
Additionally, a US fund is an offshore fund from Britain's side. Dollar strength can therefore create a UK gain taxed as income, with no American gain to match. Holding US funds suits Americans who spend in dollars. It rarely suits those whose liabilities are in pounds.
Direct Holdings That Avoid Both Regimes
Several instruments deliver a similar return to money market funds without fund status. Bank deposits and fixed-term accounts are one route, and our article on fixed rate bonds and US tax covers their timing mismatch. Short-dated gilts held directly are another, as set out in our guide to UK gilts for Americans in Britain. Each has its own quirks, yet none is a foreign investment company.
Moreover, you can instruct a manager to disable the cash sweep. Many discretionary agreements allow a no-funds mandate for US clients. Our guide to discretionary managed portfolios and US tax explains what to request. Therefore, the fix is often a single instruction, provided someone gives it before the cash arrives.
Case Study: £2 Million Parked After a Company Sale
Daniel is a US citizen in London who earns well above the top-rate thresholds in both countries. On 1 March 2024, he placed £2,000,000 of sale proceeds with a private bank. The bank swept the cash into an Irish-domiciled sterling fund with a constant £1 share price. The pound stood at $1.26, so his dollar cost was $2,520,000. He made no election.
The Income Years
The fund paid about £85,000 in 2024, £88,000 in 2025 and £60,000 in the first nine months of 2026. Britain taxed each amount at 45%. America taxed the same income at 37%, and the UK tax covered that charge by credit. However, the 3.8% investment income tax applied without credit. On roughly $300,000 of income across the period, that cost about $11,400.
The Redemption
On 1 October 2026, Daniel redeemed the holding to buy a house. The pound stood at $1.34, so he received dollars worth $2,680,000. His sterling gain was nil, and Britain charged nothing. His dollar gain, however, was $160,000, and all of it was an excess distribution.
The holding period was 945 days. Therefore, $51,810 of the gain fell in 2024, $61,799 in 2025 and $46,392 in 2026. The two earlier amounts were taxed at 37%, giving deferred tax of about $42,035, plus an interest charge of roughly $4,000. The 2026 amount added about $17,165 of ordinary tax. Furthermore, the investment income tax added $6,080.
The Outcome and the Lesson
Daniel's US tax on the redemption was therefore about $69,300, or 43% of a gain that Britain did not recognise. No foreign tax credit applied, because no UK tax existed. Moreover, his earlier returns had omitted Form 8621 entirely, so both filed tax years remained open to the IRS.
We prepared the missing forms, amended the affected returns and reported the redemption correctly. A bank deposit would still have produced a taxable currency gain. However, it would have avoided the interest charge, the open years and three sets of forms. This example is illustrative, and the exchange rates are assumed.
How TaxYork Can Help
TaxYork prepares US and UK returns for Americans whose cash sits in British accounts and money market funds. We identify every fund holding, including sweep positions that statements bury. We then compute the default-regime tax, test the elections and prepare each Form 8621. Additionally, we reconcile the FBAR and Form 8938 to the same figures.
Where earlier years are wrong, we prepare the amended returns and delinquent forms. Where returns were never filed, our catch-up filing service covers the full submission. We also prepare the UK return, so the credit claims match in both countries. Our cross-border tax preparation service covers clients before and after a large cash event.
Conclusion
Sterling money market funds are sensible products for British savers and poor ones for Americans. Britain taxes the income as interest and stops there. America adds a fund regime, a throwback rule, an interest charge and an annual form. Moreover, the dollar value of a £1 share can create a gain that only the IRS sees.
Therefore, check where your cash sits before the next statement arrives. Ask your bank or manager whether a sweep fund is in use. If one is, establish how long you have held it and whether any form has ever been filed. Acting before a redemption is far cheaper than acting after one.
Contact Us
If you hold sterling money market funds, or you suspect a manager has placed you in one, contact us to arrange a review of your US and UK filings. You can also email hello@taxyork.com or call 020 3488 8606. We will confirm which forms apply, which years need correcting and what the preparation involves.
Disclaimer
This article provides general information for US citizens and green card holders in Britain as at October 2026. It does not constitute tax, legal, investment or financial guidance for your specific circumstances. Tax rates, thresholds and rules change, and their application depends on individual facts. You should obtain professional support from a qualified US-UK tax specialist before acting on any information in this article. TaxYork accepts no liability for actions taken on the basis of this content.