Your UK Company Might Owe a US Tax Return: Form 1120-F, the Treaty Shield, and the 18-Month Rule That Can Delete Your Deductions

Thousands of non-resident entrepreneurs set up a UK limited company to sell into the United States — through Amazon FBA, a SaaS product, consulting, or e-commerce. Most assume that because the company is British, its taxes are British too. That assumption is where an expensive mistake often begins. A UK company that is "engaged in a US trade or business" can be required to file a US federal tax return — Form 1120-F — and missing it can cost far more than the tax itself.

What Form 1120-F Actually Is

Form 1120-F is the "U.S. Income Tax Return of a Foreign Corporation." The IRS defines a foreign corporation as one created or organised under the laws of a country outside the United States — which includes every UK LTD. If that company is engaged in a US trade or business during the tax year, it must file Form 1120-F to report its income and figure any US tax.

This is separate from what LLC owners deal with. It is not Form 5472 on its own, and it is not a personal return. It is the UK company's own US corporate return.

What Counts as a "US Trade or Business"

The IRS says a foreign corporation is generally engaged in a US trade or business when its US activities are "considerable, continuous and regular." There is no single bright-line test — it depends on the facts — but the IRS specifically flags situations that can create one:

• Having employees or dependent agents working in the US on the company's behalf
• Operating through a US branch
• Being a partner in a partnership that is itself engaged in a US trade or business

When a US trade or business exists, income connected to those activities becomes "effectively connected income" (ECI). ECI is taxed at the same 21% federal corporate rate as a US company — after allowable deductions.

The Treaty Shield — But You Have to Claim It

Here is the part that surprises people: many UK companies that technically must file end up owing little or no US tax, thanks to the US–UK income tax treaty. Under the treaty, a UK company's business profits are generally taxable in the US only if they are attributable to a US "permanent establishment" — broadly, a fixed place of business or a dependent agent habitually concluding contracts there. A founder selling remotely, with no US office and no US staff, often has no permanent establishment.

But the treaty does not apply itself. To claim that protection, a foreign corporation generally must still file Form 1120-F, together with Form 8833 (Treaty-Based Return Position Disclosure). The IRS is explicit on this: a foreign corporation claiming it is exempt under a treaty still files the return. Filing a "protective return" — there is a box to check on Form 1120-F — is how companies preserve the right to claim deductions and treaty benefits if the IRS later disagrees about whether a US trade or business existed.

The Deadlines

The due date depends on whether the company keeps an office or place of business in the US:

• With a US office or place of business: the 15th day of the 4th month after the tax year ends (15 April for a calendar-year company).
• Without a US office or place of business: the 15th day of the 6th month after the tax year ends (15 June for a calendar-year company).

An extension is available by filing Form 7004 before the due date.

The Real Trap: The 18-Month Rule

This is where a paperwork oversight can turn into a genuinely large bill. Under Internal Revenue Code Section 882(c)(2) and Treasury Regulation 1.882-4, a foreign corporation can only claim its deductions and credits against ECI if it files a timely, accurate return. The IRS treats the return as timely for this purpose if it is filed within 18 months of the original due date.

Miss that window, and the consequence is severe: the deductions and credits can be disallowed entirely, and the company is taxed on its gross income — its revenue, not its profit. A business with thin margins can face a tax bill larger than its actual earnings, plus late-filing penalties on top.

The IRS can waive the deadline if the company shows it acted reasonably and in good faith — but that is a discretionary relief you have to argue for, not a right. The reliable protection is simply to file on time, even when you believe no tax is due.

Don't Forget Form 5472

A foreign corporation engaged in a US trade or business may also have to file Form 5472 — the information return about reportable transactions with related parties — together with Form 1120-F. That is a separate obligation carrying its own penalty (currently 25,000 US dollars per form), so it is worth checking whether it applies to your structure.

The Bottom Line

If your UK company touches the US market in a considerable, continuous and regular way, do not assume "UK company, UK tax only." Work out whether you have a US trade or business, whether the treaty protects your profits, and — crucially — file Form 1120-F on time to lock in that protection. The cost of filing a protective or treaty return is small next to the cost of losing every deduction under the 18-month rule.

Have Questions About Your Own Situation?

Every setup is different, and whether a UK company has a US trade or business genuinely depends on the details of how it operates. If you would like to talk yours through with the MP Partner experts team — no pressure, no hard sell, just clear answers — we are happy to help.

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