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CT600 Deadline: When Is Your Company Tax Return Due?
Published 10 Jun 2026 · Updated 18 Jul 2026
Your Company Tax Return (the CT600) is due 12 months after the end of your accounting period. But the Corporation Tax itself is due earlier — 9 months and 1 day after the period ends — so you pay before you file. Plenty of first-time directors plan around the 12-month filing date and meet the earlier payment date by luck, or not at all. The practical rule: work to the 9-month date and treat the return deadline as your backstop.
When is the CT600 due?
| Obligation | Deadline |
|---|---|
| File the Company Tax Return (CT600) | 12 months after the accounting period ends |
| Pay Corporation Tax | 9 months and 1 day after the accounting period ends |
The payment deadline above is the standard one — companies with profits over £1.5 million pay Corporation Tax by quarterly instalments on a different timetable instead.
A complete return is the CT600 form + the accounts + the Corporation Tax computations. An incomplete return can be treated as not delivered, so a CT600 sent without its accounts doesn't stop the clock.
Want your exact dates from your year end? Use the deadline checker.
The first-year quirk: two CT600s, one filing date
A new company's first accounts often cover more than 12 months (incorporation to its accounting reference date), but a Corporation Tax accounting period can't exceed 12 months. HMRC splits a long first period into two: 12 months plus the remainder. That means:
- Two CT600s — one for each accounting period;
- One filing deadline — for a first period of 18 months or less, both returns are due 12 months after the end of the whole period of account;
- Two payment deadlines — each period's tax is due 9 months and 1 day after that period's end, so the first payment lands before the filing deadline.
The company tax return page covers this in full.
CT600 late filing penalties
Late return (filing dates on or after 1 April 2026): a £200 fixed penalty as soon as the return is late, rising to £400 in total once it's more than 3 months late. File late in three successive accounting periods and those figures rise to £1,000, then £2,000. On top of the fixed penalty, HMRC adds a tax-geared penalty — 10% of the unpaid tax once the return is more than 6 months late, and another 10% past 12 months (20% in total). The fixed penalty applies even if your company made a loss or owes no tax.
Late payment: interest runs on the unpaid tax from the due date — currently 7.75% (Bank of England base rate + 4%).
The dates are fixed and knowable from your year end, so the fix is a calendar, not heroics.
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Frequently asked questions
What is the CT600 due date? The CT600 due date is 12 months after the end of your accounting period. The Corporation Tax itself is due earlier — 9 months and 1 day after the period ends.
What is the CT600 submission deadline? The submission (filing) deadline is that same 12-month date. It's separate from the payment deadline: you submit the return within 12 months, but pay the tax within 9 months and 1 day.
What is the deadline for filing your Corporation Tax return? 12 months after your accounting period ends. "Corporation Tax return" and "CT600" mean the same thing — the return HMRC issues a notice to file each year.
Why is Corporation Tax due before the return? That's simply how the two deadlines are set — 9 months and 1 day for payment, 12 months for filing. Work out the tax early enough to pay on time, even though you could file later.
My company made no profit — do the deadlines still apply? The filing deadline does: a company served a notice to file must file, profit or loss. With no tax due there's nothing to pay, but the return still goes in.
Why does a new company file two CT600s? A first period of account often exceeds 12 months, and a Corporation Tax accounting period can't — so HMRC splits it in two: two returns, one filing date (for periods of 18 months or less), two payment dates.
I'm a sole trader, CIC or charity — does this apply to me? The CT600 is filed with HMRC by companies. Sole traders don't file one — your deadlines are the Self Assessment ones; a CIC does file a CT600 (it pays corporation tax); charities follow the charity tax regime. See other organisation types for your situation.
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