CT600 Late Filing Penalties: How Much They Cost and How to Avoid Them
Filing your CT600 Corporation Tax return late is one of the most expensive mistakes a UK company director can make. HMRC's penalty structure escalates quickly, and unlike some other penalties, there's very little room for appeal.
Here's exactly what happens if you miss the deadline, and how to make sure you don't.
CT600 Filing Deadlines
Your CT600 is due 12 months after the end of your accounting period. For example:
| Accounting Period Ends | CT600 Due Date |
|---|---|
| 31 March 2025 | 31 March 2026 |
| 31 December 2024 | 31 December 2025 |
| 30 June 2025 | 30 June 2026 |
Important: Your Corporation Tax payment is due earlier — 9 months and 1 day after your accounting period ends. The filing deadline and payment deadline are different.
The Penalty Structure
HMRC applies penalties automatically. There's no warning letter first.
The flat-rate penalties depend on when your filing deadline falls, not when the accounting period ended.
Filing Deadlines Before 1 April 2026
| How Late | Penalty |
|---|---|
| 1 day late | £100 |
| 3 months late | Another £100 (£200 total) |
| 6 months late | HMRC estimates your tax bill and charges 10% of the unpaid tax |
| 12 months late | Another 10% of unpaid tax |
Filing Deadlines On or After 1 April 2026
From 1 April 2026, HMRC doubled the flat-rate penalties for late CT600 returns:
| How Late | Penalty |
|---|---|
| 1 day late | £200 |
| 3 months late | Another £200 (£400 total) |
| 6 months late | 10% of unpaid tax (unchanged) |
| 12 months late | Another 10% of unpaid tax (unchanged) |
If your accounting period ended on or after 1 April 2025, your filing deadline falls on or after 1 April 2026 — meaning the higher flat-rate penalties apply to you.
Example: 12 Months Late with £5,000 Tax Due (new regime)
| Penalty | Amount |
|---|---|
| 1 day late | £200 |
| 3 months late | £200 |
| 6 months late | £500 (10% of £5,000) |
| 12 months late | £500 (10% of £5,000) |
| Total penalties | £1,400 |
That's on top of the £5,000 tax bill itself, plus interest.
Three Consecutive Late Returns
If you file late for the third time in a row, the flat-rate penalties increase further:
| Regime | 1 day late | 3 months late |
|---|---|---|
| Before 1 Apr 2026 (standard) | £100 | £100 |
| Before 1 Apr 2026 (3rd+ consecutive) | £500 | £500 |
| On/after 1 Apr 2026 (standard) | £200 | £200 |
| On/after 1 Apr 2026 (3rd+ consecutive) | £1,000 | £1,000 |
Habitual late filing becomes extremely expensive under the new regime.
Interest on Late Payment
Separate from penalties, HMRC charges interest on late tax payments from the due date (9 months and 1 day) until you pay. The current rate is 7.75% per annum (effective January 2026), calculated daily as simple interest — it does not compound. It accumulates continuously until the debt is cleared.
Note that Corporation Tax late-payment interest is deductible for Corporation Tax purposes.
Can You Appeal a Late Filing Penalty?
You can appeal if you have a reasonable excuse. HMRC accepts:
- Serious illness or bereavement
- HMRC systems being unavailable near the deadline
- Fire, flood, or other unforeseen events
- Postal delays (if filing by post)
HMRC does not accept:
- "I forgot"
- "I didn't know the deadline"
- "My accountant didn't tell me"
- "I thought it was filed automatically"
- "I was too busy"
Appeals must be made within 30 days of the penalty notice. You can appeal online via your Government Gateway account.
How to Avoid Late Filing Penalties
1. Know Your Deadline
Your CT600 is due 12 months after your accounting period ends. Mark it in your calendar — ideally with a reminder 3 months before.
2. File Early
There's no penalty for filing early and no benefit to waiting. Many directors file their CT600 within weeks of the accounting period ending.
3. Use Software That Files Electronically
Electronic filing via commercial software is instant — no postal delays, no lost forms. Your submission is acknowledged immediately with a reference from HMRC.
Taxpipe files your CT600 electronically for £59 per return. The guided wizard walks you through every step, calculates your tax automatically, and submits directly to HMRC.
4. File a Return If HMRC Has Issued a Notice
If HMRC has issued you a notice to deliver a CT600, you must file it — even for a period with no trading activity. A nil return takes minutes and avoids the initial late-filing penalty.
5. Set Up a Reminder System
HMRC sends reminders, but don't rely on them. Set your own calendar alerts for:
- 6 months before deadline: gather records
- 3 months before: prepare your return
- 1 month before: file your return
- Deadline day: final check
What If You've Already Missed the Deadline?
File immediately. The penalties escalate with time — filing one day late costs £100 or £200, but waiting three months doubles the flat penalty, and waiting six months adds a tax-geared charge.
Even if you can't pay the tax, file the return anyway. Filing penalties and payment penalties are separate. You can arrange a Time to Pay agreement with HMRC for the tax bill while avoiding further filing penalties.
Dormant Companies
A common assumption is that dormant companies never need to file a CT600. The reality is more nuanced: once you notify HMRC that a company is dormant, they should stop issuing notices to deliver returns — and you only owe a CT600 if HMRC has issued a notice to deliver. If you do receive a notice, you must file it even for a dormant period, and the same penalties apply for missing the deadline.
Taxpipe handles dormant CT600 filings — most boxes are zero and it takes only minutes.
The Bottom Line
For returns with filing deadlines on or after 1 April 2026:
| Action | Cost |
|---|---|
| Filing on time | £0 |
| Filing 1 day late | £200 |
| Filing 6 months late (£5k tax) | £900+ |
| Filing 12 months late (£5k tax) | £1,400+ |
| Using Taxpipe to file on time | £59 |
The maths is simple: spending £59 to file on time saves you hundreds or thousands in penalties.
Related: HMRC penalties for late filing
Don't risk penalties. File your CT600 with Taxpipe — guided wizard, automatic calculations, direct HMRC submission. £59 per return.