CT600 Penalties for Late Filing — What Happens If You Miss the Deadline

The CT600 Filing Deadline

Your CT600 must be filed within 12 months of the end of your company's accounting period.

Example deadlines:

Miss this deadline and HMRC starts charging penalties immediately.

The Penalty Scale

HMRC's penalty system for late CT600 filing is straightforward and unforgiving:

£100 — Filed 1 Day to 3 Months Late

The moment you miss the deadline, even by a single day, HMRC charges a flat £100 penalty. This applies whether you owe £0 or £100,000 in tax.

£200 — Filed 3 to 6 Months Late

After 3 months, HMRC adds another £100, bringing the total to £200. Again, this is a flat charge regardless of tax owed.

10% of Tax — Filed 6 to 12 Months Late

Now it gets expensive. HMRC charges 10% of the unpaid tax on top of the £200. If you owe £10,000 in corporation tax, that's an extra £1,000.

For dormant companies owing £0, the 10% penalty is £0 — but you still pay the £200 flat penalties.

20% of Tax — Filed Over 12 Months Late

A further 10% of unpaid tax is added, making it 20% total on top of the £200 flat penalties.

Repeat Offenders

If you've filed late in 3 consecutive periods, the flat penalties double:

• 1 day late: £500 (instead of £100)
• 3 months late: £1,000 (instead of £200)

Real Examples

The Payment Deadline Is Different

Don't confuse the filing deadline (12 months) with the payment deadline (9 months and 1 day). Interest on unpaid tax runs from the payment deadline, not the filing deadline.

You can file late and still avoid interest — as long as you paid on time. And you can pay late but file on time — you'll get interest but not filing penalties. They're independent.

How to Avoid Late Filing Penalties

1. Set Calendar Reminders

As soon as your accounting period ends, set reminders for:

• 6 months: Start preparing your return
• 8 months: Have your return ready
• 9 months: Pay your corporation tax
• 11 months: File if you haven't already
• 12 months: Absolute filing deadline

2. File Early

There's no benefit to waiting. You can file your CT600 as soon as your accounting period ends. Filing early doesn't change when your tax is due.

3. Use Software That's Ready Now

With HMRC's free filing service closing in March 2026, don't wait until the last minute to find alternative software. Set up your Taxpipe account now so it's ready when you need it.

4. File Even If You Can't Pay

Filing and paying are separate. If you can't afford the tax bill, file anyway. You'll avoid filing penalties (£100-£1,000+) and can arrange a Time to Pay plan with HMRC for the tax itself.

What If You've Already Missed the Deadline?

File Immediately

Every day costs you nothing extra in the first 3 months (the penalty is already £100). But once you cross the 3-month mark, it jumps to £200. File as soon as possible to stop the clock.

Pay Any Tax Owed

Interest is running from the payment deadline. The sooner you pay, the less interest accrues.

Consider Appealing

You can appeal a late filing penalty if you have a "reasonable excuse":

• Serious illness or bereavement
• Fire, flood, or other disaster
• HMRC errors or delays
• Postal delays (if filing by paper, which is rare now)

Not reasonable excuses:

• "I didn't know I had to file"
• "My accountant didn't tell me"
• "The company is dormant"
• "I was too busy"

Set Up a Time to Pay Plan

If you can't pay the tax + penalties in full, call HMRC's Business Payment Support Service (0300 200 3835) to arrange instalments.

Dormant Companies: You're Not Exempt

The most common penalty trap: directors of dormant companies who assume they don't need to file. You do. A dormant CT600 takes 15 minutes and costs £59 with Taxpipe. A late filing penalty costs £100-£200.

File Your CT600 Now

Don't let penalties stack up. Taxpipe lets you file your CT600 in about 30 minutes for £59 — far less than any penalty.

Check your estimated tax with our free calculator →