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 and a further £200 at three months, then 10% of the tax still unpaid if the return is 6 months late (18 months after the accounting period ends), rising to 20% if it is 12 months late (2 years after the period ends) (GOV.UK: Company Tax Returns: Penalties for late filing). Three late returns in a row raise each £200 to £1,000. Paying the tax late is charged separately, as 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: £200 on the first day and a further £200 at three months, then 10% of the tax still unpaid if the return is 6 months late (18 months after the accounting period ends), rising to 20% if it is 12 months late (2 years after the period ends).

In law this is one tax-geared penalty, not two: 10% of the tax unpaid 18 months after the period ends, or 20% if the return arrives more than 2 years after the period ends (Finance Act 1998, Schedule 18, paragraph 18).

The flat-rate penalties were doubled from £100 to £200 for returns with a filing date on or after 1 April 2026 (GOV.UK: increases to Corporation Tax late filing penalties).

If your return is 6 months late, HMRC writes to tell you how much Corporation Tax it thinks you owe. This is a tax determination, and you cannot appeal against it. You must pay the Corporation Tax due and file your return, and HMRC then recalculates the interest and penalties (GOV.UK: Company Tax Returns: Penalties for late filing). Paying the determined amount does not replace your return — you still need to file it.

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. The higher amounts apply when the two previous returns were also late, so filing the next return on time breaks the run.

HMRC treats persistent late filing more harshly, so a pattern of small delays becomes an expensive one. If several returns are already overdue, which overdue CT600 to file first sets out the order.

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 you can't pay the tax yet. Paying late is charged separately, as interest, while a late return triggers an automatic £200 penalty from the first day. 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 a figure is genuinely uncertain, take advice well before the deadline rather than guessing. Don't knowingly file placeholder figures just to beat the deadline: HMRC can charge a penalty for errors in a return (GOV.UK: Company Tax Returns: Making changes), and a careless inaccuracy — one where you didn't take reasonable care — can cost up to 30% of the extra tax (HMRC factsheet CC/FS7a).
• If you find a genuine mistake after filing, you can usually amend the return within 12 months of the filing deadline — see how to correct a CT600 after filing.

If you'd rather not track it by hand, prepare your CT600 with Taxley and file well before the deadline — or first take the 30-second check to see whether Taxley fits your company.

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 (GOV.UK: appeal a penalty).

A shortage of funds or leaving it to the last minute is generally not accepted as a reasonable excuse.

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 10% and 20% tax-geared 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, file straight away. Leaving it outstanding adds the second £200 at 3 months, then 10% of the unpaid tax if the return is 6 months late, rising to 20% if it is 12 months late. You must file before you can appeal (GOV.UK: Company Tax Returns: Penalties for late filing).

Can I file an estimate on time and amend it later?

Don't plan on it. A Company Tax Return should reflect figures you took reasonable care over, and HMRC can charge a penalty for careless errors. You can usually amend a genuine mistake within 12 months of the filing deadline (GOV.UK: Company Tax Returns: Making changes), but if a figure is uncertain, get advice rather than filing a guess.