Corporation Tax return (CT600) deadline

The Corporation Tax return (form CT600) is due 12 months after the end of the company's accounting period. Note the trap: the *payment* of Corporation Tax falls due 9 months and 1 day after the period end — three months earlier than the return.

What is this filing?

The CT600 reports the company's taxable profits to HMRC and calculates the Corporation Tax liability. It is filed online via HMRC's CT online service, separately from the annual accounts filed at Companies House.

When is it due?

Filing deadline: 12 months after the end of the accounting period. Payment deadline: 9 months and 1 day after the end of the accounting period. Large companies pay in quarterly instalments. For accounting periods exceeding 12 months, two CT600 returns are required.

What's the penalty for missing it?

Late return: £100 (1 day late), £100 (3 months late — same amount, second flat penalty), 10% of unpaid tax (6 months late), additional 10% of unpaid tax (12 months late). Repeated lateness escalates flat penalties to £500 each. Interest accrues on late tax from day one after the payment deadline.

How PenaltyProof helps

PenaltyProof derives the CT600 and Corporation Tax payment deadlines from the per-company `ct_period_end` setting and emails advance warnings (paid plans). Toggle on at the practice level on the Pro plan; per-company setting captured at /manage.

Frequently asked questions

Is the deadline the same as for annual accounts?

No. Accounts: 9 months after the accounting reference date. CT600: 12 months after the accounting period. They usually align but can diverge after a period change.

Can the CT600 be filed early?

Yes, and HMRC encourages it. The return can be filed once the accounts are finalised.

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