You Missed The Corporation Tax Filing Deadline By One Day. Now What?

You filed your CT600 (corporation tax return) one day late. Maybe you thought the deadline was the 31st when it was the 30th. Maybe your accountant emailed you and you opened it too late. Maybe you just forgot.

Whatever the reason, the late corporation tax filing penalty is automatic. HMRC does not send a warning letter first. There is no grace period. One day late means one penalty triggered.

Here is exactly what happens, how much it costs, and what you should do next.

The First Penalty: £100 Automatic Fine

The moment your CT600 is overdue, HMRC issues a £100 penalty. This applies to every company, no matter how small your profit or loss. Even if your corporation tax bill is zero, the £100 still stands.

A sole director Ltd company in Sheffield turning over £40,000 with £8,000 profit? £100 penalty. A 10-employee consultancy in Birmingham with £900,000 turnover? Same £100 penalty. The penalty is flat, not proportional.

HMRC sends a penalty notice to your registered office address. You do not get a phone call or an email reminder. If you have not updated your registered office with Companies House, that notice goes to your old address. You may not know you have been penalised until the debt collection letters start.

What If Your Corporation Tax Bill Was Paid On Time?

Paying your corporation tax on time does not protect you from the late filing penalty. Filing and payment are two separate obligations. You can pay your tax on 1 January and file your CT600 on 1 April. If your filing deadline was 31 March, you are one day late and the £100 penalty applies.

The reverse is also true. You can file on time and pay late. That triggers different penalties (interest on overdue tax and a 5% late payment penalty after 30 days). But the late filing penalty is about the return, not the payment.

What Happens After The First £100 Penalty?

The late corporation tax filing penalty escalates on a fixed schedule. Here is the timeline for a company with a 31 March year end and a 31 December filing deadline:

  • 1 day late (1 January): £100 penalty issued automatically.
  • 3 months late (31 March): A second £100 penalty is issued. Total so far: £200.
  • 6 months late (30 June): HMRC estimates your tax bill and charges 10% of the unpaid tax as a penalty. If your estimated bill is £20,000, that is a £2,000 penalty on top of the £200.
  • 12 months late (31 December): Another 10% of the unpaid tax. If still unpaid, that is another £2,000. Total penalties now £4,200.
  • 18 months late (30 June, year two): A further 10% penalty. Plus HMRC may escalate to tax-geared penalties of up to 100% of the tax due if they believe the delay is deliberate.

These penalties compound quickly. A single missed deadline can turn into a five-figure penalty bill if you ignore it for 12 to 18 months.

Does HMRC Ever Cancel The Late Filing Penalty?

HMRC can cancel a late corporation tax filing penalty if you have a reasonable excuse. The bar is high. HMRC defines reasonable excuse narrowly.

Examples that typically count:

  • Your company's registered office was destroyed by fire or flood in the week before the deadline.
  • You were hospitalised unexpectedly and physically could not file.
  • A serious postal strike delayed delivery of documents you needed to file.

Examples that do not count:

  • "My accountant forgot." HMRC holds the company responsible for its agents.
  • "I was too busy." HMRC hears this constantly and rejects it.
  • "I did not receive the reminder." There is no statutory requirement for HMRC to remind you.
  • "The software crashed." You are expected to have backups and alternative methods.

If you think you have a reasonable excuse, write to HMRC within 30 days of receiving the penalty notice. Use form SA370 for self assessment penalties, or write directly for corporation tax penalties. Include evidence: hospital discharge letters, insurance claims, correspondence with your software provider.

What To Do If You Miss The Deadline By One Day

Here is your action plan, in order:

1. File the return immediately. Do not wait. The penalty clock is already ticking. File today, even if the return is not perfect. You can amend it later. A filed return stops further daily penalties from accruing.

2. Pay any corporation tax due. If your return shows tax owing, pay it within 30 days of the original deadline to avoid the 5% late payment penalty. Interest will still accrue from the original due date, but you can avoid the percentage penalty.

3. Check your registered office address. Log in to Companies House and confirm your registered office is correct. HMRC sends penalty notices there. If it is wrong, update it immediately and redirect your post.

4. Review your filing process. Why did you miss the deadline? Was it a diary error? Did your accountant not flag it? Fix the root cause so it does not happen again. A second late filing within 12 months doubles the initial penalty to £200.

5. Consider whether to appeal. If you genuinely have a reasonable excuse, write to HMRC within 30 days. If you do not, pay the £100 and move on. Arguing without grounds wastes time and can trigger HMRC to review your entire compliance history.

Can You Avoid The Late Filing Penalty Entirely?

Yes. File on time. That is the only guaranteed way.

But if you know you will miss the deadline, file an estimated return before the deadline. You can file a CT600 with estimated figures, then file an amended return later. HMRC accepts estimated returns. The key is having something submitted before midnight on the deadline date.

If you use accounting software like Xero or FreeAgent, set up automated reminders 30 days, 14 days and 3 days before your filing deadline. If your accountant handles filing, agree a deadline reminder protocol in writing. Some accountants send a calendar invite for the filing date itself, not just the tax payment date.

For companies with a 31 March year end, the filing deadline is 31 December. That is a bank holiday and New Year's Eve. File early. Do not leave it to 31 December. HMRC's online systems have gone down on high-traffic days. If the system is down at 11:59pm on the deadline, you still get the penalty. HMRC does not extend deadlines for system outages.

What About The Corporation Tax Payment Deadline?

The payment deadline for corporation tax is 9 months and 1 day after your year end. For a 31 March year end, that is 1 January. Yes, New Year's Day. If you pay on 2 January, you are one day late.

Late payment triggers:

  • Interest: HMRC charges interest from the due date to the date of payment. The rate changes quarterly. As of early 2025, it is around 7.75% per year.
  • Late payment penalty: 5% of the unpaid tax if it is still outstanding 30 days after the due date. A further 5% at 6 months and another 5% at 12 months.

These are separate from the late filing penalties. You can be penalised for late filing and late payment simultaneously.

Real Example: A Manchester Freelancer Ltd

Take a freelance consultant in Manchester's Northern Quarter. She runs her business through a limited company with a 31 March year end. Her corporation tax bill is £14,720. She pays it on 2 January (one day late). She files her CT600 on 5 January (five days late).

Here is what she owes:

  • £100 late corporation tax filing penalty (first penalty).
  • Interest on £14,720 for 4 days at 7.75%: roughly £12.50.
  • No late payment penalty (paid within 30 days of due date).

Total extra cost: £112.50. Not catastrophic. But if she had left the return unfiled for 6 months, the penalties would reach £2,100 plus interest. If she left it 12 months, over £4,200.

How Holloway Davies Can Help

As ICAEW qualified accountants, we handle corporation tax filing for hundreds of UK limited companies. Our team tracks every filing deadline across our client base. We file returns weeks before the deadline, not days. We set up payment calendar reminders for every client.

If you have already missed a deadline, we can help you file immediately, assess whether you have grounds for a reasonable excuse appeal, and set up a system to prevent recurrence. We also handle the CT600 preparation and filing, so you never have to think about the deadline yourself.

Read our corporation tax guide for a full overview of your obligations. If you are just starting out, our fundamentals section covers the basics of running a limited company.

If you need to get your filing back on track, contact our team.