Corporation Tax deadlines: when to pay and file

In short: Corporation Tax has two separate deadlines. You must pay what you owe within 9 months and 1 day of the end of your accounting period, and you must file your Company Tax Return (the CT600 plus accounts and computations) within 12 months of the end of that period (GOV.UK: Company Tax Returns: Overview). If your accounts cover more than 12 months, they are split into two returns with separate payment dates, and both returns are due 12 months after the end of the accounts. Confusingly, the payment deadline comes before the filing deadline. Taxley files the CT600 to HMRC, including a return that's already late, for £39.95 with micro-entity accounts or £129 with small-company accounts, then files the accounts at Companies House once the return is paid and finished.

The payment date landing first catches a lot of directors out. If you'd rather not track both by hand, you can prepare your CT600 with Taxley and see what you owe in good time.

What are the two Corporation Tax deadlines?

You must pay any Corporation Tax due within 9 months and 1 day of your accounting period end, and file your CT600 within 12 months of it, so the payment deadline comes first. Both deadlines run from your accounting period end, not your company's incorporation date or the tax year.

To see both dates for your own period, use the Corporation Tax deadline calculator.

When is my Corporation Tax payment due?

Most small companies must pay within 9 months and 1 day of the end of their accounting period. For example, if your accounting period ends on 31 March 2026, your Corporation Tax payment is due by 1 January 2027 — and your CT600 isn't due until 31 March 2027, three months later.

It's a common trap: the money is due before the paperwork.

Large companies — broadly those with taxable profits of more than £1.5 million (a limit divided between associated companies) — pay in quarterly instalments instead, and very large companies with profits over £20 million pay earlier still (GOV.UK: pay Corporation Tax; paying in instalments). A company that wasn't large in the previous 12 months doesn't usually have to pay by instalments in its first large year, unless its profits exceed £10 million. All of this is outside the scope of a typical small company.

When is the CT600 filing deadline?

You must file your Company Tax Return within 12 months of the end of your accounting period — for a period ending 31 March 2026, that is 31 March 2027. The filing date comes after the payment date, but a late return still triggers a £200 penalty from the first day. The return includes:

• the CT600 form itself,
• your statutory accounts (FRS 105 or FRS 102 §1A for most small companies), and
• your tax computations,

all submitted to HMRC online in iXBRL format.

What if my first accounting period is longer than 12 months?

A Corporation Tax accounting period cannot exceed 12 months, so a longer first period of account is split into two: the first 12 months and the remainder, each with its own CT600 and payment date. For accounts of up to 18 months, both returns normally share one filing date: 12 months after the accounts end, not 12 months after each period (Finance Act 1998, Schedule 18, paragraph 14).

For example, accounts from 1 April 2025 to 30 September 2026 give these dates:

If HMRC's notice to file a return arrives late, that return is due 3 months after the notice if that is later (CT603 notice to deliver). The free long accounting period calculator works out both periods and their dates from your accounts start and end dates. See first company accounts: why you may need two CT600s for how the two returns work.

What are the penalties for filing late?

HMRC charges £200 as soon as a Company Tax Return is late and another £200 at three months, then 10% of the tax still unpaid if the return is 6 months late (18 months after the accounting period ends), rising to 20% if it is 12 months late (2 years after the period ends) (GOV.UK penalties):

The flat-rate penalties were uprated from £100 to £200 for return filing dates on or after 1 April 2026, and rise to £1,000 each if the return is late three times in a row. Paying late is separate and accrues interest on the unpaid amount, so work your figure out early — our Corporation Tax rates and Marginal Relief guide shows how, and the free Corporation Tax calculator gives a quick estimate. For the full schedule and how to appeal, see what happens if you file your CT600 late.

Frequently asked questions

Is the Corporation Tax payment deadline before the filing deadline?

Yes. Payment is due at 9 months and 1 day, while the return isn't due until 12 months — so you generally pay before you file. You need to work out what you owe in time to pay it.

Are both CT600s due on the same date when my accounts cover more than 12 months?

Usually, yes. When accounts covering up to 18 months are split into two Corporation Tax periods, both returns are due 12 months after the end of the accounts. The payment dates differ: each period's tax is due 9 months and 1 day after that period ends.

What date does my accounting period end?

For most companies it matches the "accounting reference date" shown on Companies House, which usually defaults to the end of the month of your incorporation anniversary. You can check it on the public register.

Do I have to file online?

Yes. Company Tax Returns must be filed online with HMRC, with the accounts and computations attached as iXBRL. HMRC's own online filing service closed on 31 March 2026, so you now need commercial software or an agent; paper returns are only accepted if you have a reasonable excuse or are filing in Welsh (GOV.UK: Closure of HMRC's file-your-accounts-and-tax-return service).

What's the penalty for filing my CT600 late?

£200 the day it's late and another £200 if it's more than three months late, then 10% of the tax still unpaid if the return is 6 months late (18 months after the accounting period ends), rising to 20% if it is 12 months late (2 years after the period ends). Late payment is separate and accrues interest.

Do large companies also pay at 9 months and 1 day?

No. Large companies — broadly those with taxable profits over £1.5 million, a limit divided between associated companies — pay in quarterly instalments instead, and very large companies with profits over £20 million pay earlier still (GOV.UK: paying in instalments).

Taxley prepares your CT600 and iXBRL accounts and computations from your figures, and automatically splits a long first period into two returns. It doesn't pay HMRC for you — that remains your responsibility by the payment deadline. Not sure it suits your company? Take the 30-second check.