What Happens If You Miss Your CT600 Filing Deadline?
Your CT600 is due 12 months after the end of your accounting period. Miss it, and the penalties start immediately.
The Penalty Schedule
The fixed penalties depend on when your CT600 filing deadline falls — not when you eventually file.
For returns with a filing deadline before 1 April 2026:
| How Late | Penalty |
|---|---|
| 1 day | £100 |
| 3 months | Additional £100 (£200 total) |
| 6 months | 10% of unpaid tax (based on HMRC's estimate if you haven't yet filed) |
| 12 months | Additional 10% of unpaid tax |
If you're late 3 years in a row, the £100 penalties increase to £500 each.
For returns with a filing deadline on or after 1 April 2026:
| How Late | Penalty |
|---|---|
| 1 day | £200 |
| 3 months | Additional £200 (£400 total) |
| 6 months | 10% of unpaid tax (based on HMRC's estimate if you haven't yet filed) |
| 12 months | Additional 10% of unpaid tax |
If you're late 3 years in a row, the £200 penalties increase to £1,000 each.
The tax-geared penalties (at 6 and 12 months) are applied to the unpaid tax outstanding at those points — so if you've paid your tax on time, those charges can be zero or much reduced even if you file the return late.
Interest on Late Payment
Corporation Tax is due 9 months and 1 day after your accounting period ends — that's 3 months BEFORE the CT600 filing deadline.
If you pay late, HMRC charges interest at the Bank of England base rate + 4% (this formula has applied since 6 April 2025). The rate has been 7.75% since 9 January 2026.
Unlike most penalties, late-payment interest on Corporation Tax is tax-deductible — it reduces your taxable profits in the period it's paid.
Example: 6 Months Late
Your company has an April 2025 year-end:
- CT600 due: 30 April 2026 (on or after 1 April 2026 — new penalty regime applies)
- Corporation Tax due: 1 February 2026
If you file 6 months late (October 2026) with £10,000 tax owed:
| Item | Amount |
|---|---|
| Late filing penalty (1 day) | £200 |
| Late filing penalty (3 months) | £200 |
| Tax-geared penalty (10% at 6 months) | £1,000 |
| Late payment interest (9 months at 7.75%) | £581 |
| Total penalties + interest | £1,981 |
That's nearly 20% of your tax bill — gone to penalties.
What If You Can't Pay?
HMRC offers Time to Pay arrangements. Contact them before the payment deadline if you can:
- Phone: 0300 200 3835
- Be ready with: how much you owe, why you can't pay, a proposed payment plan
A Time to Pay agreement helps manage the interest on unpaid tax, but it does not stop the late-filing penalties. Those only stop once you actually file the CT600. If you can't pay the tax, file the return as soon as possible anyway — a filed return with unpaid tax is a much better position than an unfiled one.
What If You Have Nothing to File?
A dormant company only needs to file a CT600 if HMRC has issued it a notice to deliver one. If you've received that notice, it's quick — just tick the dormant boxes and submit.
File a dormant CT600 with Taxpipe → (the wizard auto-detects dormant companies and fills everything for you)
How to Never Miss a Deadline
- Know your dates — check your CT603 notice from HMRC
- File early — don't wait until the last week
- Use software — Taxpipe validates your return before submission, so no back-and-forth rejections eating up your time
- Set a reminder — 3 months before the deadline
File Now
If your CT600 is overdue, file it today. Every day late is another day of interest.
Taxpipe — File your CT600 in under an hour. £59, no subscription.