How to file a CT600 (Company Tax Return)

If HMRC sends your company a notice to deliver a Company Tax Return, you need to file a CT600, with your accounts and tax computation, within 12 months of the end of the accounting period. Since HMRC’s own filing service closed, that means using commercial software.

Last checked against GOV.UK on 29 September 2026.

A4O Accounts prepares the CT600 from your books and files it with HMRC. See the CT600 software, or get started free.

Who has to file

Your company must file a Company Tax Return if it gets a “notice to deliver a Company Tax Return” from HMRC. The notice (form CT603) tells you the period the return should cover and the date it’s due.

You still have to file if the company made a loss or has no Corporation Tax to pay. If your company isn’t trading, see our guide to dormant company accounts.

What a Company Tax Return includes

The CT600 form is only one part. A complete return is:

• The CT600 itself, which works out your profit or loss for Corporation Tax and the tax you owe.
• Any supplementary pages you need, such as CT600A for loans to participators.
• Your company’s accounts for the period.
• A tax computation showing how you got from the profit in the accounts to the taxable profit on the return. Profit for Corporation Tax is worked out differently from the profit in your accounts, so the two figures are often not the same.

Filed online, the computation must be in iXBRL, and for most companies the accounts must be too. Filing software does the tagging for you.

What you need before you start

1. Your company’s Corporation Tax UTR. This is the 10-digit Unique Taxpayer Reference on letters from HMRC.
2. The Government Gateway user ID and password that has Corporation Tax on the company’s business tax account. See the HMRC details you need for Corporation Tax.
3. The accounts for the period, finished and approved.
4. The tax computation, with anything that affects it: capital allowances, losses brought forward from earlier years, and any figures carried over from last year’s return.

Accounting periods and return periods

A return covers an accounting period for Corporation Tax, and that can’t be longer than 12 months. It’s usually the same as the financial year in your accounts, but not always.

It can differ in the year you set up the company, or when your accounts cover more than 12 months. In that case you file two returns to cover the period of the accounts: one for the first 12 months, and one for the rest. If the dates HMRC holds are wrong, tell HMRC before the original filing date, or you may get a late filing penalty.

Deadlines

The payment deadline comes first, so work out the tax early even if you file later. See Corporation Tax deadlines for the details.

Filing now HMRC’s service has closed

HMRC’s free “File your accounts and Company Tax Return” service closed on 31 March 2026. GOV.UK now says to file online using commercial software. Paper returns are only accepted if you have a reasonable excuse or are filing in Welsh. Our guide to the closure covers what changed.

HMRC keeps a list of software it accepts returns from. When choosing, check the software covers the whole return (the CT600, any supplementary pages, the accounts and the computation) and not just one part.

Your accounts still go to Companies House separately, usually 9 months after your year end. From April 2028 that also has to be done through software. See filing company accounts with Companies House and the move to software-only filing.

After you file

Keep a copy of the return, the accounts and the computation exactly as filed.

If you find a mistake, you can send an amended return through your software. You must usually do this within 12 months of the filing deadline. After that, if you’ve overpaid you may still be able to claim overpayment relief. If you’ve underpaid, GOV.UK says to tell HMRC through its online disclosure service. HMRC can also check your return and may charge a penalty for errors.

Late filing penalties

For returns due on or after 1 April 2026:

If your return is late three times in a row, the £200 penalties rise to £1,000 each. If you have a reasonable excuse you can appeal, but you must file the return first.

Doing it in software

A4O Accounts prepares the accounts and the CT600 from the same books. It sends the CT600 to HMRC with the accounts and tax computation tagged in iXBRL, including the CT600A and CT600L supplementary pages where you need them, and it can file amended returns. You enter your Government Gateway user ID and password when you file, and they aren’t stored. You can send a test submission first. See CT600 software for what it covers. The step-by-step is in preparing and filing a CT600.

More in this series: all the CT600 guides.