Corporation Tax deadlines explained: 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. Confusingly, the payment deadline comes before the filing deadline.

What are the two Corporation Tax deadlines?

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:

If your return is late three times in a row, the £100 penalties increase to £500 each. Paying late is separate and accrues interest on the unpaid amount.

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.

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.