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). Confusingly, the payment deadline comes before the filing deadline.

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?

There are two deadlines and they fall on different dates: 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. The payment deadline comes first.

These run from your accounting period end, not your company's incorporation date or the tax year.

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 2025, your Corporation Tax payment is due by 1 January 2026 — and your CT600 isn't due until 31 March 2026, three months later. It's a common trap: the money is due before the paperwork.

Very large companies (generally those with profits over £1.5 million) pay in quarterly instalments instead, but that's 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. 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. A company's first "period of account" is often longer — so it gets split into two Corporation Tax periods: the first 12 months, and the remainder. You then file two CT600 returns, each with its own payment and filing deadlines.

What are the penalties for filing late?

HMRC charges escalating penalties for a late Company Tax Return (GOV.UK penalties):

The flat-rate penalties were uprated from £100 to £200 for return filing dates on or after 1 April 2026. If your return is late three times in a row, the flat penalties rise to £1,000 each. 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. 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.

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. Paper returns are only accepted in very limited circumstances.

What's the penalty for filing my CT600 late?

£200 the day it's late, another £200 if it's more than three months late, then 10% of the unpaid tax at six months and a further 10% at twelve. Late payment is separate and accrues interest.

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.