Introduction: Why a UK Demerger Can Be Tax-Free in London and Taxable in Washington
A UK demerger splits one company into two and hands shareholders shares in the new business, usually without any British tax at all. However, an American shareholder answers to a second tax system with its own, stricter test. Consequently, the same shares can arrive tax-free from HMRC and as a taxable dividend from the IRS.
The question is no longer academic. Unilever completed its UK demerger of The Magnum Ice Cream Company on 6 December 2025. Consequently, thousands of Americans in Britain now report it on their 2025 US returns. Furthermore, the extended filing deadline of 15 October 2026 is only weeks away.
At TaxYork we prepare both returns for high-net-worth shareholders, and demergers produce some of the most expensive errors we correct. In our experience, the damage rarely comes from the UK side. Instead, it comes from treating the American return as if it simply follows the British one.
What a UK Demerger Actually Is
A UK demerger separates businesses held in one company or group into two or more independently owned companies. The shareholders end up owning both halves directly. Listed groups use the technique to release value, and private families use it to divide a business between siblings.
Britain offers three main routes. The statutory route relies on an "exempt distribution" under the Corporation Tax Act 2010. Alternatively, a capital reduction demerger or a liquidation demerger under section 110 of the Insolvency Act 1986 can achieve the split. Both rely on the reconstruction rules in the capital gains legislation. The HMRC Company Taxation Manual at CTM17250 introduces the statutory regime.
Why Americans Face Two Separate Tests
Every UK demerger must satisfy British rules to be tax-free in Britain. Meanwhile, the United States applies section 355 of the Internal Revenue Code to the same transaction. However, section 355 was written for American spin-offs, not British demergers.
The two tests overlap, but they are not the same. Specifically, section 355 demands five years of active business on both sides, a genuine business purpose and no "device" for distributing earnings. Therefore, a UK demerger that HMRC clears in weeks can still fail in Washington.
The June 2026 Consultation That Widens the Gap
On 23 June 2026 the government published a consultation on modernising the taxation of distributions and repayments of capital. It closes on 14 September 2026. Notably, it proposes the most significant liberalisation of demerger relief in decades.
Three proposals matter most. First, the residence requirement for every company involved would disappear. Second, references to "trade" would become "activity", opening the relief to investment companies. Third, restrictions on later sales would apply only for five years.
For British shareholders this is good news. For Americans it is a warning. An investment company can never meet the American active business test. Consequently, the reform would create more UK demerger transactions that are tax-free in Britain and taxable in America.
How Britain Taxes a UK Demerger
British law treats a qualifying UK demerger as a non-event for the shareholder. Nevertheless, the conditions are strict, and the treatment changes sharply when they fail.
The Statutory Exempt Distribution
On a statutory UK demerger, an exempt distribution escapes income tax entirely. Accordingly, the shareholder receives the new shares without any dividend charge. The distribution can be direct, where the company hands over shares in a subsidiary. Alternatively, it can be indirect, where a new company issues shares to the members in return for a business or subsidiary.
Four conditions apply. Each relevant company must currently meet a residence condition, which a South African or American company fails. The companies must be trading companies or members of a trading group. The distribution must benefit the trading activities, and it must not form part of a scheme with tax avoidance as a main purpose. Additionally, the company must make a return within thirty days of the distribution, as CTM17260 explains.
Reorganisation Treatment and Base Cost Apportionment
For capital gains tax, section 192 of the Taxation of Chargeable Gains Act 1992 treats an exempt distribution as a share reorganisation. Consequently, no disposal occurs. The Capital Gains Manual at CG45620 confirms the point.
Instead, the shareholder splits the original base cost between the old and new shares. The split follows their market values on the first day of dealing in the new shares, and helpsheet HS285 sets out the method. Furthermore, the new shares inherit the acquisition date of the old ones.
When the Exemption Fails: The Anglo American Example
The residence condition has real teeth. Anglo American demerged its South African platinum business on 31 May 2025. Because that company was not UK resident, no exempt distribution was available.
Anglo American's published letter to UK shareholders states that the distribution was taxable as income. The maximum effective rate was 39.35 per cent of its value. In its illustrative example, a holder of 0.4 per cent received shares worth about £14.8 million and faced income tax of roughly £5.8 million. Therefore, the identical commercial transaction produced wildly different results depending on where the demerged company sat.
That result is precisely what the June 2026 consultation targets. Nevertheless, until the law changes, any UK demerger of a company that fails the residence condition should be assumed taxable for British individuals. The rates appear on the GOV.UK dividend tax page.
Chargeable Payments and the Five-Year Watch
Relief on a UK demerger comes with a tail. Section 1086 of the Corporation Tax Act 2010 targets any "chargeable payment" made within five years of an exempt distribution. The recipient pays tax on it as income. Moreover, the payer gets no deduction for it, as CTM17290 records.
In practice, the rule catches value extracted from either company other than by normal dividends. Therefore, shareholders in a private UK demerger should treat unusual payments during the five years with considerable caution.
How the United States Taxes the Same Shares
American law starts from the opposite presumption. Any distribution of property to shareholders is taxable unless a specific rule says otherwise. For a UK demerger, that rule is section 355.
The Section 355 Requirements
Section 355 imposes five core conditions. The distributing company must control the spun-off company, meaning eighty per cent of voting power and of every other class. Furthermore, it must generally distribute all of it. Both companies must actively conduct a trade or business immediately afterwards, and each business must have run actively throughout the preceding five years.
Additionally, the transaction must serve a genuine corporate business purpose. It must not be a device for distributing earnings and profits, and the historic shareholders must keep a continuing interest in both companies. Regulation 1.355-3 defines the active business test, and holding investment property generally fails it.
Listed groups plan for section 355 from the outset. For example, Unilever's US tax filing states that its demerger was intended to qualify under sections 355 and 368. Private companies, in contrast, rarely consider it at all.
Basis Allocation Under Section 358 and Form 8937
Where section 355 applies to a UK demerger, the shareholder recognises no income. Instead, section 358 spreads the existing tax basis across the old and new shares in proportion to their fair market values. The holding period of the new shares includes that of the old ones.
Companies report the basis effect on Form 8937. However, Unilever did not prescribe a fixed percentage. Instead, it suggested that one reasonable approach uses the closing London prices on 8 December 2025, the first day the new shares traded. Accordingly, each American shareholder must make and document the allocation personally.
Lot matters too. American law allocates basis block by block, for each purchase at a different date or price. British law pools the shares instead. Consequently, a UK demerger leaves an American investor with two base cost ledgers that never match.
What Happens When Section 355 Fails
A UK demerger that fails section 355 is a distribution under section 301. The shareholder receives a dividend equal to the fair market value of the new shares, to the extent of the company's earnings and profits. The new shares then take a basis equal to that value.
A UK company normally pays qualified dividends under the treaty, so the top rate is twenty per cent. Furthermore, the net investment income tax adds 3.8 per cent. Cash paid in place of fractional shares is treated separately, as a sale of the fraction.
The Four Outcomes and the Foreign Tax Credit
Every UK demerger lands in one of four combinations. The foreign tax credit behaves very differently in each, and IRS Publication 514 governs the mechanics.
Tax-Free on Both Sides
Most listed groups design every UK demerger for this outcome. Neither country taxes the receipt. Nevertheless, the shareholder must still allocate basis on both sides, report the new holding and keep the two ledgers accurately. Any later sale then produces two different gains, in two currencies, calculated from two different starting points.
Exempt in Britain, Taxable in America
This is the dangerous combination. The UK demerger is exempt, so HMRC charges nothing. Meanwhile, the IRS treats the shares as a dividend or, in a split between shareholders, a taxable exchange. No British tax exists to credit.
Worse, the problem repeats later. The British base cost in the new shares is a small apportioned slice of the original cost, while the American basis equals full market value. On a future sale, Britain therefore taxes a large gain and America a small one. Consequently, the later British tax can exceed the American tax it should offset, and the excess strands.
Taxable in Britain, Tax-Free in America
The Anglo American UK demerger pattern creates the mirror image. Britain charges income tax on the value of the shares, while section 355 may treat the same receipt as a non-event. The British tax remains a creditable foreign income tax, but in the year it is paid there is no matching American income.
In practice, that tax can only reduce American tax on other foreign income in the right category. Any excess carries back one year and forward ten. Therefore, an investor without substantial other foreign income can lose much of the credit permanently.
Two Base Costs That Never Reconcile
Even the best outcome leaves permanent divergence. British base cost is pooled in sterling. American basis is tracked lot by lot, in dollars, at historical exchange rates. Additionally, a British investor sells from the pool, while an American can identify specific lots. For this reason we keep a separate American ledger for every client who holds demerged shares.
Private Company Demergers for American Owners
A private UK demerger carries larger sums and fewer safeguards than a listed one. Moreover, they tend to fail the American test for reasons no British adviser checks.
Partition Demergers and Section 1248
A classic family partition gives different shareholders different businesses. One sibling takes the trading company, and another takes the property company. Britain relieves the split through the reconstruction rules, usually after clearance.
America sees something else. A non-pro-rata split of a controlled foreign corporation falls within Regulation 1.367(b)-5, which can require an income inclusion measured by section 1248. Furthermore, if section 355 fails, the American shareholder's surrender of old shares becomes a taxable exchange. Section 1248 can then recharacterise part of the gain as a dividend.
Consider a US citizen in London who owns half of a UK group worth £15 million. In the split, she takes a £7.5 million property company. Her American basis is £1.1 million, so her gain is £6.4 million, about $8.6 million at 1.35. At up to 23.8 per cent, that approaches $2.05 million, with no British tax to credit.
The Active Business Test and Investment Companies
In a family UK demerger, the property side is usually where section 355 fails. A portfolio let through managing agents is an investment, not an active trade, for American purposes. Consequently, a partition that hands one sibling the property company often fails for everyone.
The June 2026 consultation would make this worse, not better. Once "trade" becomes "activity", British relief will reach demergers of investment businesses that America will never treat as tax-free. Therefore, American shareholders in family investment companies should model any proposed UK demerger before HMRC clearance is even sought.
New Companies, New Forms
Every new company created by a UK demerger in which an American holds at least ten per cent creates a new Form 5471 obligation. Non-filing carries a $10,000 penalty per year. The Schedule O ownership disclosure also records the reorganisation itself. Additionally, related reporting often shifts: our guide to scheme of arrangement takeovers covers the parallel issues when a UK company is acquired rather than divided.
For listed shares, account reporting changes too. Shares held in a UK nominee account form part of that account's value for the FBAR. Certificated shares held directly on the register are not an account at all, but they still count for Form 8938.
Case Study: A Unilever Holding and a £99,500 Mistake
The following case is a composite drawn from our files, with figures adjusted for confidentiality. The fifteen per cent value split is an illustrative assumption; each shareholder must use actual first-day prices.
The Facts
A dual US-UK national in Hampstead held 60,000 Unilever shares in a UK nominee account. She bought 30,000 in March 2012 at £21.50 and 30,000 in June 2019 at £46.00. On the demerger she received 12,000 Magnum shares, one for every five Unilever shares.
A generalist preparer drafted her 2025 US return on extension. The draft reported the new shares as a dividend of about $418,000, their approximate market value. At 23.8 per cent, that produced American tax of roughly $99,500, with no British tax to credit, because the UK demerger was exempt.
What We Corrected
We removed the dividend entirely, relying on the section 355 position in Unilever's Form 8937. Next, we allocated basis lot by lot. Her 2012 lot cost £645,000, or $1,019,100 at 1.58, and her 2019 lot cost £1,380,000, or $1,752,600 at 1.27. Fifteen per cent of each gave the Magnum shares an American basis of $415,755.
Britain produced a different figure. Her section 104 pool of £2,025,000 gave the Magnum shares a British base cost of £303,750. Consequently, she now holds one sterling pool and two dollar lots, and we recorded both ledgers before filing. The corrected return reached the IRS well before 15 October 2026.
The Later Sale
She sold the Magnum shares in February 2026 for £330,000. Britain charged capital gains tax on a £26,250 gain, less the £3,000 annual exempt amount, at 24 per cent under the current CGT rates. That produced £5,580.
The American gain was $26,445, measured against her allocated dollar basis. The British tax, worth about $7,477, fully covered the American regular tax. Only the 3.8 per cent net investment income tax of roughly $1,005 remained. Therefore, correct allocation saved her about $99,500 in 2025 and kept the 2026 sale clean.
How TaxYork Can Help
Every UK demerger demands both regimes applied to the same shares, at the same date, in two currencies. Furthermore, the errors compound quietly for years until a sale exposes them.
Reporting Listed Demergers Correctly
We review each issuer's Form 8937 and the IRS instructions for it, allocate basis lot by lot and maintain separate British and American ledgers. Additionally, our US tax return preparation for expats team handles extensions and amended returns where earlier years went wrong.
Modelling Private Demergers Before Clearance
For private groups, we run the section 355 analysis before HMRC clearance is sought. Consequently, families see the American cost of each structure while they can still change it. Our tax treaty optimisation work addresses the credit position on later disposals.
Keeping the Reporting Clean
New companies mean new Forms 5471, and new holdings change account reporting. Accordingly, our FBAR and FATCA compliance service updates every filing that a UK demerger touches.
Conclusion
A UK demerger is designed to be invisible to British shareholders. However, for an American a UK demerger is never invisible, because section 355 applies its own test. A failure produces a dividend or a taxable exchange with no British tax to credit. Listed demergers like Unilever's usually qualify, yet they still require careful basis allocation. Private partitions often fail outright.
The June 2026 consultation will widen the gap by extending British relief to investment companies that can never meet the American active business test. Therefore, model any proposed demerger before it happens, and check every past one before 15 October 2026.
Contact Us
Our specialists prepare US and UK returns for high-net-worth shareholders and business owners on both sides of the Atlantic. Therefore, speak to us before you vote on a demerger, sign a partition agreement or file a return that reports one.
Email hello@taxyork.com or call 020 3488 8606 to book a consultation. Additionally, you can review our full range of cross-border tax services. Further background is available from the Chartered Institute of Taxation, the ICAEW, the American Institute of CPAs and IRS Publication 550 on investment income.
Disclaimer
This article provides general information on the UK demerger rules and United States tax treatment as at September 2026. It does not constitute tax advice, and no reader should act on it without professional guidance applied to their own circumstances. The case study uses illustrative figures. Tax law changes frequently, and the June 2026 consultation may alter the British rules. TaxYork accepts no liability for any action taken in reliance on this article.