Corporation tax filing deadlines

The two key deadlines

Every accounting period has two separate corporation tax deadlines:

Deadline	When
Tax payment	9 months and 1 day after the accounting period end
CT600 filing	12 months after the accounting period end

The payment deadline is earlier than the filing deadline — this is the most common mistake. Many companies wait until their accountant files the return to pay, but by then interest may be accruing.

Example: Period ends 31 March 2026.

• Tax payment due: 1 January 2027 (9 months and 1 day).
• CT600 filing due: 31 March 2027 (12 months).

Setting reminders in AccBooks

AccBooks automatically creates calendar reminders for both deadlines when you set your financial year end. View them at Corporation tax → Deadlines and click Add to calendar to export them to your phone or Outlook.

Quarterly instalments for large companies

If your taxable profits exceed £1.5 million (divided by associated companies), you must pay quarterly instalments:

Instalment	Due date
1st	6 months and 13 days into the accounting period
2nd	9 months and 13 days into the accounting period
3rd	12 months and 13 days into the accounting period
4th	15 months and 13 days into the accounting period

Each instalment is 25% of the estimated annual tax liability.

AccBooks flags quarterly instalment obligation under Corporation tax → Payment schedule if your profits approach the threshold.

Very large companies

Companies with profits over £20 million (divided by associated companies) pay even earlier:

• 2nd month of the accounting period (14 days in).
• 5th, 8th, and 11th months.

This regime is shown separately in AccBooks’ payment schedule.

Penalties for late filing

HMRC charges automatic penalties for late CT600s:

Late by	Penalty
Up to 3 months	£100 flat rate
3–6 months	£200 flat rate
6–12 months	10% of tax unpaid
Over 12 months	Additional 10% of tax unpaid

Persistent late filers face escalating penalties. AccBooks shows your filing status prominently on the corporation tax page — never miss a deadline.

Interest on late payments

HMRC charges interest on unpaid corporation tax from the due date. The late payment interest rate is the Bank of England base rate plus 2.5% (currently around 7.5% per annum). Interest accrues daily, so even a short delay can be costly.

What if you can’t pay?

If you’re unable to pay corporation tax in full by the deadline, contact HMRC’s Payment Support Service before the due date to arrange a Time to Pay agreement. HMRC is generally sympathetic if you make contact proactively.

AccBooks can help you model your cashflow to identify payment issues early — see Reports → Cashflow forecast.

First accounting period rules

For newly incorporated companies, the first accounting period often runs for more than 12 months (from incorporation to the first chosen year-end). HMRC splits this into two accounting periods — AccBooks handles this automatically under Corporation tax → First period split.