How to file a nil CT600 tax return with HMRC

By DormantFile · Updated 27 May 2026

If your dormant company is registered for Corporation Tax, you need to file a CT600 return with HMRC every year — even if the company owes no tax. This is called a nil CT600 because every figure on the return is zero.

What is a CT600?

A CT600 is the Corporation Tax return that all UK companies registered for Corporation Tax must file with HMRC. It reports the company's income, gains, and tax liability for each accounting period. For a dormant company, the return shows zero income, zero gains, and zero tax — hence "nil CT600."

What you need

• Unique Taxpayer Reference (UTR) — a 10-digit number issued by HMRC when your company registered for Corporation Tax.
• HMRC Government Gateway credentials — the user ID and password for your company's Government Gateway account.
• Accounting period dates — the start and end date of the period being filed. Your accounting reference date determines these.

Note: these credentials are separate from your Companies House authentication code, which you need for filing accounts.

When is the deadline?

The CT600 must be filed within 12 months of the end of the accounting period.

For example, if your accounting period ends on 31 March 2026, the CT600 is due by 31 March 2027.

This is longer than the Companies House accounts deadline (9 months). We cover both deadlines in our guide on dormant company filing deadlines.

What happened to CATO?

Until March 2026, HMRC offered a free online tool called CATO (Corporation Tax Online) for filing CT600 returns directly. HMRC closed CATO on 31 March 2026. Companies now need to use third-party commercial software. We cover all the alternatives in our guide on what to do now that CATO has closed.

How to file

Using DormantFile: Add your company, enter your UTR and accounting period, then file your CT600. You enter your HMRC Gateway credentials at the point of submission — they are used once to authenticate with HMRC and are never stored. The whole process takes under 2 minutes. See how it works.

DormantFile handles dormant company filings only. If your company has traded or has transactions to report, you need accounting software or a professional.

Using other software: Commercial accounting packages like Xero, FreeAgent, or TaxCalc can file CT600 returns. These are designed for trading companies and are typically £100+/year.

Hiring an accountant: An accountant can file on your behalf, typically charging £80--£150 per company per year.

Is your data secure?

Your HMRC Gateway credentials are used at the moment of filing only. They are transmitted directly to HMRC over an encrypted TLS connection and discarded from memory immediately. They are never written to any database. We explain our approach in detail on our security page.

Do you definitely need to file?

Only if your company is registered for Corporation Tax. Many dormant companies — especially those that have never traded — may not be registered. Check our guide on whether you need a CT600 for a dormant company.

Do not forget your accounts

If you need a CT600, you almost certainly also need to file annual accounts with Companies House. We handle both filings from one dashboard — see our guide on how to file dormant company accounts. Plans start at £19/year.

Key points

• A nil CT600 confirms your dormant company owes no Corporation Tax.
• Deadline: 12 months after the end of your accounting period.
• You need your UTR and HMRC Gateway credentials — separate from your Companies House authentication code.
• CATO (HMRC's free filing tool) closed on 31 March 2026.
• DormantFile submits your nil CT600 directly to HMRC via their GovTalk API.