How to File a CT600 Company Tax Return in 2026 (HMRC's Free Service Has Closed)

HMRC's free "File your accounts and Company Tax Return" service closed on 31 March 2026. Since 1 April 2026, every limited company has to file its CT600 through commercial software. That includes small and dormant companies whose directors used to do it themselves for free. Paper returns are only accepted with a reasonable excuse.

This guide explains what HMRC needs from you now, when it's due, what being late costs, and how to file a CT600 without an accountant.

What you have to send HMRC

A Company Tax Return is three documents, all filed together in iXBRL format:

1. The CT600 form: your taxable profit, the tax due and any reliefs
2. The tax computation: how you got from accounting profit to taxable profit (add-backs such as depreciation and entertaining, capital allowances, losses)
3. The company accounts: profit and loss account and balance sheet, with prior-year figures

HMRC says to check that your software can produce and file all three. Some tools only do the form.

CT600 deadlines

Payment comes before filing. A company with a 31 December 2025 year end must pay by 1 October 2026 and file by 31 December 2026. Large companies (profits over £1.5 million) pay by quarterly instalments instead.

First accounting period longer than 12 months? You file two CT600s: one for the first 12 months and one for the rest.

Late filing penalties have doubled

The fixed penalties for a late Company Tax Return are now:

• 1 day late: £200
• 3 months late: another £200
• 6 months late: HMRC estimates your tax and adds 10% of the unpaid tax
• 12 months late: another 10% of any unpaid tax
• Late three times in a row: the £200 penalties rise to £1,000 each

Interest runs on late-paid tax from the day after the payment deadline. Six months after the filing deadline, HMRC can issue a determination of your tax that you can't appeal. You have to file the return to replace it.

How much Corporation Tax will you pay?

• 19% if profits are £50,000 or less (small profits rate)
• 25% if profits are over £250,000 (main rate)
• Between the two, marginal relief gives an effective rate in between

The limits are divided by the number of associated companies and reduced for short accounting periods. Our Corporation Tax calculator works out your figure.

Do dormant companies need to file a CT600?

If HMRC has sent a "notice to deliver a Company Tax Return", you must file one, even if the company was dormant. Otherwise, tell HMRC the company is dormant and you won't usually need to file. Dormant companies still file accounts at Companies House every year (see filing company accounts).

How to file a CT600 step by step

1. Gather your figures: income, expenses, assets, liabilities, and last year's comparatives.
2. Have your references ready: company registration number and Corporation Tax UTR (10 digits, on HMRC letters).
3. Prepare the accounts for the period.
4. Work through the computation: add back non-deductible costs (depreciation, client entertaining, fines), claim capital allowances and use any losses brought forward.
5. Check the CT600 boxes, including the directors' loan section (CT600A) if a director owed the company money at the year end.
6. Approve and submit. Keep the HMRC acknowledgement as your receipt.
7. Pay by the payment deadline if you haven't already.
8. File the accounts at Companies House. It's a separate filing with its own deadline.

Filing your CT600 without an accountant

Many small companies, such as contractors, single-director businesses, property companies and dormant companies, have simple enough affairs to file their own return. It's worth paying an accountant if you have a group, overseas branches, R&D claims or anything unusual.

File your CT600 with AccLedger

AccLedger is HMRC-recognised software for the whole Company Tax Return:

• File one CT600 for £39 + VAT, with no subscription. Enter your figures, review the computation, and AccLedger produces the CT600, iXBRL computation and iXBRL accounts and files them to HMRC. If HMRC rejects it, fix it and resubmit at no extra cost. A later amended return for the same period is included too.
• Keep your books in AccLedger on the limited company plan (£10 a month), and the CT600 is prepared from your ledger, with periods over 12 months split automatically.
• Then file the same accounts at Companies House for £19 + VAT. AccLedger checks the balance sheet matches what HMRC accepted.

Start a 30-day free trial — no card needed.

Frequently asked questions

Can I still file a CT600 for free with HMRC? No. HMRC's free service closed on 31 March 2026. You now need commercial software.

When is my CT600 due? 12 months after your accounting period ends. The tax itself is due 9 months and 1 day after the period ends.

What is the penalty for a late CT600? £200 if it's a day late, another £200 at 3 months, then 10% of unpaid tax at 6 and 12 months. Repeat offenders pay £1,000.

Can I file a CT600 on paper? Only if you have a reasonable excuse for not filing online, or you're filing in Welsh.

Do I need to file accounts with my CT600? Yes. HMRC needs the accounts and computation in iXBRL with the CT600. Companies House needs its own copy of the accounts separately.

Can I file a CT600 without an accountant? Yes, if your affairs are straightforward. AccLedger files the CT600, computation and accounts for £39 + VAT.

This article is general guidance based on HMRC rules as of October 2026 and is not tax advice.