What is a CT600?

By Ben Davies · Updated 5 October 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 HMRC sends your dormant company (one with no money in or out of its books all year) a notice to deliver a Company Tax Return (its formal letter asking for one, form CT603), you must file a CT600 for that period — even though the company earned nothing and owes no tax. The return shows no profit and no tax to pay, so it is called a nil CT600.

Not filing when required triggers automatic penalties from HMRC, starting at £200 for being one day late — or £100 where the return fell due before 1 April 2026.

What you need

What makes a return due is HMRC's notice to deliver, not the UTR (the company's 10-digit tax reference). Every company gets a UTR after incorporation, on HMRC's CT41G letter, so having one doesn't mean a return is due — and not finding yours doesn't mean one isn't. 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 HMRC login)
• Your accounting period dates

How to file

HMRC's free filing service — "File your accounts and Company Tax Return", known as CATO — closed on 31 March 2026. A CT600 now has to go through commercial software; paper is allowed only with a reasonable excuse or for a return in Welsh. 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. For the full picture — deadlines, nil returns, and catching up on missed years — read the complete guide to filing a CT600 for a dormant company.

Read the full guide: How to file a nil CT600 tax return with HMRC