Corporation tax return deadline: when the CT600 is due and what late filing costs

Your Company Tax Return is due twelve months after the end of the accounting period it covers. The flat-rate late filing penalty is £200, rising to £400 — amounts that changed for returns whose filing date falls on or after 1 April 2026. A great deal of published material still quotes the superseded figures.

What is the corporation tax return deadline?

Twelve months after the end of the accounting period the return is made for. For an accounting period ending 31 March, the CT600 is due by 31 March the following year.

This is the standard case and it covers the great majority of companies. It is not, however, the whole of the statutory rule.

Is twelve months always the deadline?

Not always — it is the earliest the deadline can ever be, never the latest.

Paragraph 14(1) of Schedule 18 to the Finance Act 1998 sets the filing date as the last day of whichever of four periods is the last to end:

• (a) twelve months from the end of the period for which the return is made;
• (b) if the company's relevant period of account is not longer than 18 months, twelve months from the end of that period;
• (c) if the relevant period of account is longer than 18 months, 30 months from the beginning of that period;
• (d) three months from the date on which the notice requiring the return was served.

Branch (a) carries no condition, so it applies to every company without exception. Because paragraph 14(1) selects the period last to end, the true filing date can never be earlier than branch (a) gives — the twelve-month figure is a floor.

The practical consequence is reassuring: working to twelve months can only ever have you filing sooner than the law requires, never later. But a company with a long period of account, or one served with a late notice, may genuinely have more time than the standard rule suggests.

What is the penalty for a late Company Tax Return?

A flat-rate penalty under paragraph 17 of Schedule 18. Paragraph 17(2): "The penalty is— (a) £200, if the return is delivered within three months after the filing date, and (b) £400, in any other case."

Those are the amounts for a return whose filing date falls on or after 1 April 2026.

Why do some sources still say £100?

Because £100 was correct until recently, and a great deal of published material has not been updated.

Section 265(1) of the Finance Act 2026 substituted the paragraph 17 amounts: £100 became £200, and £200 became £400. The superseded pair is not folklore — it is instrument-sourced, quoted in the substitution words of the amending provision itself. It was the right answer for years, which is exactly why it persists in guidance, summaries and third-party explainers.

Quoting £100 as the current first-tier penalty is now wrong for any return with a filing date on or after 1 April 2026. Quoting £200 for a return whose filing date fell before that boundary is equally wrong, and wrong in the more expensive direction — it doubles the customer's stated exposure.

Which returns keep the old £100 and £200 amounts?

Those whose filing date fell before 1 April 2026. Section 265(2) of the Finance Act 2026 is explicit: "The amendments made by subsection (1)(a) and (b) have effect in relation to a failure to deliver a company tax return for which the filing date is on or after 1 April 2026."

Three things follow, and each is a trap.

The trigger is the filing date, not the delivery date. A return whose filing date fell before 1 April 2026 keeps the pre-amendment amounts — £100 and £200 at paragraph 17(2), £500 and £1000 at paragraph 17(3) — however late it is eventually delivered. Filing it in 2027 does not move it onto the new figures.

The trigger is the filing date, not the accounting period end. These are different dates, and the filing date is the one paragraph 14(1) defines.

The two usually track each other, but not always. On the standard twelve-month branch, accounting periods ending on or before 31 March 2025 have filing dates before 1 April 2026 and stay on the superseded amounts. But branches (b) to (d) can produce a filing date on or after 1 April 2026 for an accounting period that ended before that boundary — and then the amended amounts apply. Reasoning from the accounting period end alone will get those cases wrong.

Is the penalty £200 and then another £200?

No. Paragraph 17 imposes one flat-rate penalty per failure, whose amount steps up at the three-month mark.

It is a single penalty of £200 that becomes a single penalty of £400 once the return is more than three months past the filing date. It is not £200 charged now and a further £200 charged later.

Both readings arrive at £400 as the ceiling, so the arithmetic often matches. The structure does not — and the structure is what determines how the amount behaves, how it is charged, and what is being appealed against.

What is a third successive failure?

An escalation that replaces the ordinary amounts rather than adding to them. Paragraph 17(3): "The amounts are increased to £1000 and £2000 for a third successive failure."

So a third-successive-failure penalty is £1000, stepping up to £2000 past the three-month mark. It is not £1000 on top of £400.

The conditions in paragraph 17(3)(a) to (d) are cumulative — all four must hold:

• the company is within the charge to corporation tax for three consecutive accounting periods, and at no time between the beginning of the first and the end of the last is it outside the charge;
• a company tax return is required for each of them;
• the company was liable to a paragraph 17 penalty for each of the first two; and
• it is again liable for the third.

A gap in the charge to corporation tax anywhere across the run breaks the sequence.

What is the tax-geared penalty?

A second, separate penalty under paragraph 18, charged as a percentage of unpaid tax and stated by the Act to be "in addition to any flat-rate penalty under paragraph 17".

Paragraph 18(2): "The penalty is— (a) 10 per cent. of the unpaid tax, if the return is delivered within two years after the end of the period for which the return is required, and (b) 20 per cent. of the unpaid tax, in any other case."

Paragraph 18 was verified as unamended by the Finance Act 2026 — only the flat-rate amounts moved.

When does the tax-geared penalty arise?

At the later of two dates. Paragraph 18(1) catches a company that fails to deliver "(a) within 18 months after the end of that period, or (b) if the filing date is later than that, by the filing date".

So for a company on the standard twelve-month filing date, the tax-geared charge bites at 18 months after the accounting period end — six months after the filing date, not at some fixed interval from it.

Is the tax-geared penalty 10% and then another 10%?

No — the same stepped-single-penalty structure as paragraph 17. It is one penalty at 10% that becomes one penalty at 20%, not 10% followed by a further 10%.

The measure for the step is also easy to get wrong: the two-year period in paragraph 18(2)(a) runs from the end of the accounting period, not from the filing date. Widely circulated framings that describe a percentage at "6 months late" and another at "12 months late" match neither the instrument's trigger nor its structure.

What counts as "unpaid tax"?

Paragraph 18(3) defines it as the tax payable for the accounting period that remains unpaid on the date liability arises under paragraph 18(1).

Paragraph 18(4) disregards certain deferred relief — under sections 458 and 464B of the Corporation Tax Act 2010 — when computing that figure.

The base is therefore fixed at the moment the liability arises, which is why paying the tax down after that date does not retrospectively shrink the penalty.

Is filing the CT600 the same as paying corporation tax?

No, and the deadlines are three months apart. The tax is due nine months and one day after the accounting period ends; the return is generally due twelve months after it.

The tax falls due before the return that computes it is legally required. The consequences diverge sharply too: the return carries the penalties on this page, while late payment of the tax carries no penalty at all — only interest.

What is not covered here?

Several genuine cases sit outside this page and are named rather than quietly folded in:

• Branch (c) — a period of account longer than 18 months, where the filing date is 30 months from the beginning of that period.
• Branch (d) — the late-notice case, three months from service of the notice requiring the return.
• Branch (b) wherever the relevant period of account ends later than the accounting period itself.
• Returns required for a period that is not an accounting period, under paragraphs 5(3) and 5(4). This page's twelve-months-from-the-accounting-period-end rule does not describe those at all.

Each needs facts — period of account start and end, or the notice service date — that are not derivable from public register data.

How do I file a CT600?

Online to HMRC, with accounts and computations, for the accounting period covered. Work back from the accounting period end, and treat twelve months as the working deadline while remembering it is a floor: it is safe precisely because it can only ever be early.

Related

• Pay corporation tax — due nine months and one day after the period end, with no late-payment penalty
• Annual accounts and late filing penalties — the Companies House filing on the same period, due at nine months
• Adequate accounting records — the underlying record-keeping duty
• Pillar hub: UK company tax deadlines (placeholder)
• Companies House deadline checker — enter your company number to see the accounting period end both deadlines count from