Corporation Tax payment deadline: 9 months and 1 day explained

Corporation Tax has an unusual feature that catches out new company directors every year: the tax is due before the return. Most small companies must pay their Corporation Tax 9 months and 1 day after the end of their accounting period, while the CT600 return is not due until 12 months after.

The payment deadline

For companies with taxable profits of up to £1.5 million, Corporation Tax is due 9 months and 1 day after the end of the accounting period.

Because the tax depends on the accounts, in practice the accounts and tax computation need to be finished several weeks before this date – not at the Companies House filing deadline, which is usually later.

Large companies pay in instalments

Companies with taxable profits over £1.5 million (shared between companies in a group) pay by quarterly instalments, starting in the seventh month of the accounting period. Very large companies, with profits over £20 million, pay even earlier. A company that is large for the first time is not required to pay by instalments for that year, provided its profits do not exceed £10 million.

Late payment interest

HMRC charges interest on Corporation Tax paid late, from the day after the due date until payment. The late payment rate is set at the Bank of England base rate plus 4%, following the increase from 6 April 2025. Interest on late payments is not deductible for tax purposes.

If a client cannot pay in full, talk to HMRC before the deadline. A Time to Pay arrangement can stop the problem growing.

Accounting periods longer than 12 months

A company’s accounting period for Corporation Tax can never be longer than 12 months. If the company prepares accounts for a longer period – common with first accounts – there are two Corporation Tax periods: the first 12 months, and the remainder. Each has its own payment deadline and its own CT600. Missing the first one is a classic error.

Practical tips for practices

1. Work back from the payment date, not the filing date. Aim to finish small company accounts at least a month before the 9-month payment deadline so the client has time to pay.
2. Tell clients the figure and the date in writing, with HMRC’s payment reference.
3. Watch first-year companies with long first periods.
4. Keep the period end accurate. In Deadlio, the payment date is calculated from the next period end, and moves automatically when you record that the accounts are done.

Related guides

• CT600 return filing deadline
• Annual accounts filing deadline
• VAT return deadlines
• All deadline guides

This guide is general information for accountancy practices and is not a substitute for checking the current rules on GOV.UK. Last reviewed October 2026.