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.

What it actually costs

For CT600 returns with a filing date on or after 1 April 2026, HMRC doubled the flat penalty: £200 on day one (was £100), £400 at three months (was £100). For companies with three successive late returns, the penalties rise further to £1,000 and £2,000. These penalties are in addition to interest on any late-paid tax, which runs from the day after the payment deadline at HMRC's published rate. A client in their third year of late filing therefore faces a £1,000 fixed penalty, a further £2,000 at three months, plus 10% of unpaid tax at six months — all compounding on the same liability.

Why this catches practices out

• The payment deadline (9 months + 1 day) arrives three months before the filing deadline — many clients are blindsided because the return is not yet due.
• Accounting periods exceeding 12 months require two CT600 returns: one for the first 12 months and one for the remainder.
• After a period-end change the new deadline may not align with the Companies House accounts deadline, creating a divergence that spreadsheets miss.

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.

What are the CT600 penalties from 1 April 2026?

For returns with a filing date on or after 1 April 2026: £200 on day one, £400 at three months. For companies that have filed late for three successive periods: £1,000 on day one, £2,000 at three months. Tax-based penalties (10% of unpaid tax) apply at six and twelve months regardless of successive-failure status.

When is Corporation Tax itself due?

The payment deadline for small companies is 9 months and 1 day after the accounting period end — three months before the CT600 filing deadline. Large companies (profits over £1.5 million) pay in quarterly instalments.

Does PenaltyProof track both the CT600 filing and the CT payment deadline?

Yes. Both deadlines are derived from the per-company ct_period_end setting. PenaltyProof alerts at 90, 60, 30, 14, and 7 days before each date on paid plans.

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