How to file a nil CT600 tax return with HMRC

By Ben Davies · Updated 5 October 2026

We file your accounts and tax return for £19/year

If HMRC sends your dormant company a notice to deliver a Company Tax Return, you need to file a CT600 return with HMRC, even if the company owes no tax. For a dormant company (no money in or out of its books all year — Companies House calls these 'significant accounting transactions') 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 a UK company files with HMRC when HMRC sends it a notice to deliver one. 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 HMRC sends after your company is incorporated, on a letter called a CT41G.
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 (the 6-character code Companies House posts to the company's registered office), 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 service, "File your accounts and Company Tax Return" — known as CATO (Company Accounts and Tax Online) — which filed a company's accounts and its CT600 together. HMRC closed it on 31 March 2026. A CT600 now has to go through third-party commercial software; paper is only for a reasonable excuse or a return in Welsh. We explain what changed 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 about two minutes. See how it works.

DormantFile files nil returns only — for dormant companies, and for non-trading ones whose only activity is something like a loan being repaid. If your company traded or has taxable profit to report, you need an accountant.

Compared with an accountant: An accountant typically charges £80--£150 per company per year to file the same return. DormantFile files it, with the Companies House accounts, from £19/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 HMRC has sent the company a notice to deliver a Company Tax Return. No notice means no return. A notice that does arrive must be answered, even for a dormant period. 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 file both from one dashboard, and the accounts attached to your CT600 are the same ones we file at Companies House — 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.