HMRC Corporation Tax penalties can turn a small oversight into a big expense. File your CT600 just one day late, and you're immediately facing a fixed penalty — even if your company owes no tax at all.

Here's exactly what happens when you miss your CT600 deadline, how much it costs, and what you can do about it.

The penalty timeline

HMRC penalties follow a strict timeline. There's no grace period, no warnings, and no "we'll let it slide this once."

Two penalty regimes: before and after 1 April 2026

HMRC doubled the flat filing penalties for returns with a filing deadline on or after 1 April 2026. The tax-geared penalties (at 6 and 12 months) are unchanged between regimes.

Penalty stage	Filing deadline before 1 Apr 2026	Filing deadline on/after 1 Apr 2026
Day 1 (deadline missed)	£100	£200
3 months late	+£100 (total £200)	+£200 (total £400)
3rd consecutive late return	£500 per flat penalty	£1,000 per flat penalty

To find which regime applies to you: take your accounting period end date, add 12 months — that's your filing deadline. If it falls on or after 1 April 2026, the higher rates apply.

Day 1: Fixed penalty

Miss your deadline by even one day, and HMRC automatically issues a fixed penalty. This applies to every company, even if:

You have no Corporation Tax to pay
Your company is dormant
You made a loss

3 months late: Second fixed penalty

At exactly 3 months after your deadline, HMRC adds another fixed penalty — bringing your total flat penalties to £200 (old regime) or £400 (new regime).

6 months late: 10% of unpaid tax

This is where penalties get serious. HMRC estimates your Corporation Tax liability and charges 10% of that estimate as an additional penalty.

There is no minimum charge. If your company owes no tax — because it is dormant, made a loss, or has a nil liability — the 10% penalty is simply £0. You only face a tax-geared penalty if you actually have unpaid Corporation Tax.

12 months late: Another 10% penalty

HMRC adds a further 10% of the estimated unpaid tax on top of everything that came before. Again, if no tax is owed, this charge is £0.

Real penalty examples

The examples below use the current rates (filing deadline on or after 1 April 2026).

Example 1: Dormant company (£0 tax due)

1 day late: £200
3 months late: £200 + £200 = £400
6 months late: £400 + £0 (10% of £0) = £400
12 months late: £400 + £0 (another 10% of £0) = £400

Total penalties for a company with zero tax liability: £400

The 10% charges apply only to unpaid tax — so no unpaid tax means no tax-geared penalty.

Example 2: Small trading company (£2,000 tax due)

1 day late: £200
3 months late: £400
6 months late: £400 + £200 (10% of £2,000) = £600
12 months late: £600 + £200 (another 10% of £2,000) = £800

Example 3: Profitable company (£15,000 tax due)

1 day late: £200
3 months late: £400
6 months late: £400 + £1,500 (10% of £15,000) = £1,900
12 months late: £1,900 + £1,500 = £3,400

Consecutive late returns

If your company files late three years in a row, the flat penalties step up significantly. Under the current regime, each flat penalty is £1,000 (instead of £200) for a third consecutive late return. Under the old pre-April 2026 regime, the equivalent step-up was to £500 (instead of £100). The tax-geared 10% charges remain the same regardless of whether it is a first or third consecutive late return.

When you can appeal penalties

HMRC accepts appeals based on "reasonable excuse." What counts:

Reasonable excuses HMRC accepts:

Serious illness (you or close family)
Computer/software failure (if you can prove it)
Postal delays (with Royal Mail proof)
Fire, flood, or theft of essential records

What HMRC doesn't accept:

"I forgot the deadline"
"I was too busy"
"My accountant didn't remind me"
"I didn't know about the deadline"
"I couldn't afford to pay"

How to avoid penalties altogether

1. Know your exact deadline

Check your accounting period end date and add 12 months. Set multiple calendar reminders.

2. File early

Don't wait until the deadline. File 2–3 weeks early to allow for any problems.

3. Use filing software

Good software tracks deadlines and reminds you automatically.

At Taxpipe, we automatically track your deadlines and send reminders well before they're due. Our system won't let you file late accidentally, and filing costs just £59 — a fraction of even the smallest penalty.

HMRC penalties are harsh but predictable. Miss your deadline by one day, pay £200. Miss it by a year, pay hundreds more. The choice is yours, but the calendar doesn't care about your excuses.

File on time, every time. Your bank account will thank you.

Don't risk penalties — file now

Filing late costs at least £200. Filing with Taxpipe takes about 15 minutes and costs £59. Do the maths.

File your CT600 now →

CT600 Penalties: What Happens If You File Late?