What happens if you file your CT600 late?

In short: If your CT600 is late, HMRC charges an automatic £200 penalty the day it's overdue, a further £200 if it's more than three months late, then tax-geared penalties of 10% of the unpaid tax at 6 and 12 months (GOV.UK). Filing late three years in a row raises the flat penalties to £1,000 each. Paying the tax late is a separate charge that accrues interest.

Miss the filing deadline — 12 months after your accounting period ends — and the first penalty is automatic, with no tax due required for it to apply. The way to avoid it is simply to file on time, even if you can't pay yet.

What are the penalties for filing a CT600 late?

The penalties escalate the longer the return is outstanding, and they apply even if your company owes no tax. Here is the full schedule:

The flat-rate penalties were uprated from £100 to £200 for return filing dates on or after 1 April 2026. The two tax-geared penalties at 6 and 12 months are based on HMRC's own estimate of your Corporation Tax, which is why filing a return — even a rough one you later amend — is better than filing nothing.

What if you file late three times in a row?

If your Company Tax Return is late for three consecutive accounting periods, the two flat-rate penalties increase from £200 to £1,000 each. HMRC treats persistent late filing more harshly, so a pattern of small delays becomes an expensive one. Filing on time — even with estimated figures you correct afterwards — resets the clock.

What if you can't pay the Corporation Tax on time?

Paying late and filing late are two different things, charged separately — so if money is tight, still file on time. Your Corporation Tax is due 9 months and 1 day after your accounting period ends (before the filing deadline), and unpaid tax accrues interest from that date. If you can't pay, contact HMRC about a Time to Pay arrangement, but file the return regardless to avoid the automatic late-filing penalty on top. See our Corporation Tax deadlines guide for how the two deadlines line up.

How do you avoid a late-filing penalty?

File the CT600 by the deadline — 12 months after the accounting period ends — even if the figures aren't perfect or the tax isn't paid yet. Practical steps:

• Note both deadlines as soon as the period ends (pay at 9 months + 1 day, file at 12 months).
• Prepare the return early so a rejection leaves time to fix and resubmit.
• If you're not ready, file your best estimate and amend it later — an amended return avoids the penalty that filing nothing would trigger.

If you'd rather not track it by hand, prepare your CT600 with Taxley and file well before the deadline.

Can you appeal a CT600 late-filing penalty?

Yes. You can appeal a late-filing penalty if you have a reasonable excuse — HMRC gives examples such as a serious illness or a genuine failure of HMRC's online service. You normally have 30 days from the penalty notice to appeal, and you'll need to have filed the outstanding return. A shortage of funds or leaving it to the last minute is generally not accepted as a reasonable excuse (GOV.UK: appeal a penalty).

Frequently asked questions

How much is the penalty for filing a CT600 one day late?

£200, charged automatically the day the return becomes late (up from £100 for filing dates before 1 April 2026). It applies even if your company owes no Corporation Tax.

Is the late-filing penalty the same if I owe no tax?

The flat-rate penalties still apply even with no tax due — £200, then a further £200 after three months. The 6- and 12-month penalties are a percentage of unpaid tax, so those are nil if there's nothing to pay, but the flat penalties are not.

What's the difference between filing late and paying late?

Filing is delivering the CT600 (deadline: 12 months after the period end). Paying is settling the Corporation Tax (deadline: 9 months and 1 day). They have different deadlines and different charges — a late-filing penalty and interest on late-paid tax — so you can be caught by one without the other.

Can I still file after HMRC has issued a penalty?

Yes — and you should, immediately. The flat penalties don't grow once charged, but leaving the return outstanding pushes you toward the 6- and 12-month tax-geared penalties. File it as soon as you can, then appeal the penalty if you had a reasonable excuse.