Guides

CT600 Deadlines: When to File and When to Pay

Updated 10 Jun 2026

Two deadlines, in an unintuitive order: your Corporation Tax is due 9 months and 1 day after your accounting period ends, but the CT600 return itself isn't due until 12 months after. You pay before you file. Plenty of first-time directors plan around the later date and meet the payment deadline by luck or not at all — so the practical rule is to work to the 9-month date and treat the return deadline as the backstop.

The deadlines

The payment deadline above is the standard one — companies with profits over £1.5 million pay 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.

What if you miss the deadlines?

Late return (filing dates from 1 April 2026): a £200 fixed penalty, rising to £400 in total if the return is more than 3 months late — with much higher figures for repeat offenders, and tax-geared penalties (10%, then 20% of unpaid tax) once a return is more than 6 and 12 months late.

Late payment: interest runs on the unpaid tax from the due date — currently 7.75% (Bank of England base rate + 4%).

All in the "stay on top of it" spirit: the dates are fixed and knowable from your year end, so the fix is a calendar, not heroics.

Frequently asked questions

Why is the payment due before the return? That's just how the two deadlines are set — 9 months and 1 day for payment, 12 months for filing. Compute 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.

Is the deadline ever earlier than 12 months? The 12-month filing deadline runs from the accounting period end; what catches people isn't an earlier filing date but the earlier payment date and, in year one, the two-CT600 split.

I'm a sole trader — does this apply to me? No. Sole traders don't file a CT600 — your deadlines are the Self Assessment ones. See sole trader accounts.

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