What is a CT600?
By DormantFile · Updated 28 March 2026
A CT600 is the Corporation Tax return that UK companies file with HMRC. It reports a company's income, gains, and how much Corporation Tax it owes for an accounting period.
Why it matters for dormant companies
If your dormant company is registered for Corporation Tax, you must file a CT600 every year — even though it earned nothing and owes no tax. This is called a nil CT600 because every figure on the return is zero.
Not filing when required triggers automatic penalties from HMRC, starting at £100 for being one day late.
What you need
If your company has a UTR from HMRC, you almost certainly need to file. If you have never received a UTR and never registered for Corporation Tax, you probably do not — see our guide on whether you need a CT600.
To file a nil CT600, you need:
- Your company's UTR
- HMRC Government Gateway credentials
- Your accounting period dates
How to file
HMRC's free CATO service closed on 31 March 2026, so you now need commercial software to submit. We handle nil CT600 filing alongside your dormant accounts to Companies House — plans start at £19/year.
Your HMRC Gateway credentials are used once at the point of submission and never stored. See our security page for details.
Read the full guide: How to file a nil CT600 tax return with HMRC