Corporation Tax late filing penalties: the 2026 increase

Corporation Tax late-filing penalties increased for returns with a filing date on or after 1 April 2026. The first-day penalty doubled from £100 to £200, with further increases for returns filed at three months and for companies with three successive failures. Penalties apply on top of interest on any unpaid tax.

What is this filing?

A Corporation Tax late-filing penalty (CT600 penalty) is charged by HMRC when a company fails to file its Corporation Tax return by the 12-month deadline after the accounting period end. The penalty is separate from any interest on late-paid tax and from the tax liability itself. The 2026 increases apply to returns whose filing date falls on or after 1 April 2026 — earlier returns are assessed under the old £100/£100/10%/10% schedule.

When is it due?

CT600 filing deadline: 12 months after the end of the accounting period. For a company with a 31 March 2025 period end, the CT600 is due 31 March 2026 — so the new penalty scale applies to any return filed late after that date. Tax payment deadline: 9 months and 1 day after the period end (earlier than the return).

What's the penalty for missing it?

For CT600 returns with a filing date on or after 1 April 2026 (gov.uk): £200 on day one, £400 at three months. For companies that have filed late for three successive accounting 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.

What it actually costs

A £200 penalty on day one is qualitatively different from the old £100: it lands before most clients have even noticed the return is late. For a habitual late-filer in their third consecutive late year, the combined exposure is £1,000 (day one) + £2,000 (three months) + 10% of unpaid tax (six months) + 10% again (twelve months) — plus interest accruing daily from the payment deadline. The total is commonly several times the original unpaid liability.

Why this catches practices out

• The trigger is the filing date, not the accounting period end — a return with a period end before 1 April 2026 but filed late after that date incurs the new higher penalty.
• Clients who have been habitually late for two prior periods are one further late return away from the £1,000/£2,000 enhanced scale.

How PenaltyProof helps

PenaltyProof derives CT600 filing deadlines from the per-company ct_period_end setting and emails advance warnings at 30, 14, and 7 days before the deadline (paid plans). A £200 first-day penalty is not a warning — the system only works if the alert goes out well before the due date.

Frequently asked questions

Does the £200 penalty apply to all CT600s or only new ones?

The new penalty scale applies to CT600 returns whose filing date is on or after 1 April 2026, as confirmed by HMRC guidance. Returns with a filing date before 1 April 2026 are assessed under the previous £100/£100/10%/10% schedule, even if filed late after that date.

What counts as 'three successive accounting periods' for the higher penalty?

A company has filed late for three successive periods if each of its three most recent CT600 returns was filed after the 12-month deadline. The £1,000/£2,000 enhanced flat penalties apply on the next late return.

Is this penalty in addition to interest on unpaid tax?

Yes. The late-filing penalty is separate from late-payment interest, which runs from the day after the Corporation Tax payment deadline (9 months and 1 day after period end) at HMRC's current rate.

Related deadline guides