What Happens If You Miss Your CT600 Filing Deadline?

Your CT600 is due 12 months after the end of your accounting period. Miss it, and the penalties start immediately.

The Penalty Schedule

How Late	Penalty
1 day	£100
3 months	Additional £100 (£200 total)
6 months	HMRC estimates your tax + 10% penalty on unpaid tax
12 months	Additional 10% of unpaid tax

If you're late 3 years in a row, the £100 penalties increase to £500 each.

Corporation Tax is due 9 months and 1 day after your accounting period ends — that's 3 months BEFORE the CT600 filing deadline.

If you pay late, HMRC charges interest at the Bank of England base rate + 2.5%. As of 2025, that's around 7%.

Your company has a March 2025 year-end:

If you file 6 months late (September 2026) with £10,000 tax owed:

Item	Amount
Late filing penalty (1 day)	£100
Late filing penalty (3 months)	£100
HMRC tax estimate penalty (10%)	£1,000
Late payment interest (9 months at ~7%)	£525
Total penalties + interest	£1,725

That's 17% of your tax bill — gone to penalties.

HMRC offers Time to Pay arrangements. Call them BEFORE the deadline:

A Time to Pay agreement stops penalties accumulating — but you still pay interest.

Even dormant companies must file a CT600. It takes 5 minutes — just tick the dormant boxes and submit.

File a dormant CT600 with Taxpipe → (the wizard auto-detects dormant companies and fills everything for you)

Know your dates — check your CT603 notice from HMRC
File early — don't wait until the last week
Use software — Taxpipe validates your return before submission, so no back-and-forth rejections eating up your time
Set a reminder — 3 months before the deadline

If your CT600 is overdue, file it today. Every day late is another day of interest.

Taxpipe — File your CT600 in under an hour. £59, no subscription.