CT600: The Company Tax Return Form Explained

What the CT600 is

The CT600 is HMRC's Company Tax Return form. It reports the company's taxable profit or loss for one accounting period and the Corporation Tax due on it. A complete return has three parts:

• the CT600 form itself, plus any supplementary pages that apply
• the company's statutory accounts for the period
• the tax computations that turn accounting profit into taxable profit

GOV.UK points out that the profit or loss for Corporation Tax is different from the profit or loss shown in your annual accounts. The tax computations make those adjustments.

Each return covers one accounting period for Corporation Tax, which cannot be longer than 12 months. If your accounts cover a longer period (common with a company's first accounts), you must file two returns to cover them.

Who must file a CT600

A company must file a Company Tax Return if it gets a notice to deliver a Company Tax Return from HMRC. It must file even if it made a loss or has no tax to pay. That rule applies in the same way to a UK Ltd owned and run by someone living abroad.

A new company must also tell HMRC within 3 months of starting its first accounting period once it becomes active. HMRC sends the 10-digit company UTR to the registered office address, and you need it to file. Our UTR number guide explains how to find it.

CT600 filing and Corporation Tax payment deadlines

There are two different deadlines, and the payment one comes first. Many first-time directors mix them up.

Example: for an accounting period ending 31 March 2026, the Corporation Tax is due by 1 January 2027, the accounts are due at Companies House by 31 December 2026, and the CT600 is due by 31 March 2027.

Companies with profits between £1.5m and £20m pay by quarterly instalments instead, and those over £20m follow the very large company rules. HMRC charges interest on late payment. Our compliance deadline calendar works out these dates for your year end.

Corporation Tax rates the return calculates

For the financial years from 1 April 2023, including 2025 and 2026, the Corporation Tax rates are 19% for profits up to £50,000 and 25% for profits over £250,000. Profits in between pay the main rate reduced by marginal relief, using a standard fraction of 3/200. The limits are divided by the number of associated companies and reduced for short accounting periods. Good CT600 software handles the marginal relief calculation for you.

CT600 supplementary pages

The main CT600 covers most small companies. Some companies must also complete supplementary pages. HMRC's CT600 publication (updated 1 April 2026) lists these:

For an owner-managed company the page to watch is CT600A, which covers loans from a close company to participators, such as a director who is also a shareholder. If your director's loan account was overdrawn at the year end, talk to an accountant about the tax treatment before filing.

Accounts and computations in iXBRL

HMRC does not accept a PDF of your accounts as part of a normal online return. Its XBRL guide says it is compulsory for companies to send their Company Tax Returns online using iXBRL for accounts and computations. iXBRL (inline eXtensible Business Reporting Language) is a readable document with machine-readable tags behind each figure.

HMRC's manual lists a few exceptions. Non-resident companies without a UK establishment and unincorporated charities, clubs and societies may send accounts as a PDF, and a dormant company may send a copy of the most recent accounts prepared. The PDF exception is aimed at non-resident companies, so do not assume it covers a UK Ltd just because its director lives abroad; check with your accountant.

HMRC's free filing service closed on 31 March 2026

Until 2026 many small companies used HMRC's free "File your accounts and Company Tax Return" service. That service closed on 31 March 2026. From 1 April 2026 HMRC says you should use commercial software to file annual accounts and Company Tax Returns with HMRC. The same guidance links to a list of software suppliers; HMRC does not provide a free replacement.

The closure also ended the joint route that sent accounts to Companies House at the same time. You still have to file annual accounts with Companies House separately. See our micro-entity accounts guide for what small companies send there and the software-only rules from April 2028.

You now have three practical options:

• buy or subscribe to CT600 software that produces iXBRL accounts and computations, and file yourself
• use an accountant or agent who files through their own software
• use a filing service such as ours, which prepares and submits the return for you

Paper returns are only allowed if you have a reasonable excuse or are filing in Welsh.

CT600 late filing penalties

Late filing penalties doubled for returns with a filing date on or after 1 April 2026. The current penalties are:

If the return is late 3 times in a row, the £200 penalties are increased to £1,000 each. HMRC's policy paper on the increase confirms the earlier amounts were £100 and £500. These penalties apply even when the company has no tax to pay.

Companies House charges its own, separate penalties for late accounts, starting at £150 for a private company, so a late year can cost you twice.

Amending a CT600

You can change a return after filing it. GOV.UK says you must usually make any amendments within 12 months of the filing deadline, using commercial software or by writing to or sending a paper return to the Corporation Tax office. HMRC can check a return for errors and charge a penalty for inaccuracies, so correct mistakes as soon as you find them.

Dormant companies and notices to deliver

A company is usually dormant for Corporation Tax when it has stopped trading and has no other income, or is a new company that has not started trading. You can tell HMRC it is dormant, and HMRC then does not expect returns unless it sends a notice.

If you do receive a notice to deliver a Company Tax Return, you must still file one online. The return shows HMRC that the company was dormant for the period. Ignoring the notice triggers the same £200 penalties as for a trading company.

Dormant for HMRC and dormant for Companies House are separate tests. A dormant company still files accounts and a confirmation statement at Companies House. Our dormant company accounts service covers the Companies House side.

Filing yourself or using Borderless Filings

You can file a CT600 yourself with commercial software if you are comfortable with tax adjustments and iXBRL tagging. For a simple company the software cost may be modest. The risk is in the details: wrong accounting periods, missed supplementary pages, capital allowances and marginal relief.

With our company accounts and CT600 service, our filing partner prepares the accounts and computations in iXBRL from your records, checks them with you, submits the return, and tracks it in your portal with reminders for next year. We are not a firm of chartered accountants or a registered tax adviser, and we do not give tax advice: for tax planning, loans to participators or residence questions, speak to a qualified professional. See pricing and how it works.

Sources

• GOV.UK: Company Tax Returns
• GOV.UK: Pay Corporation Tax
• GOV.UK: Company Tax Returns, penalties for late filing
• HMRC: Increases to Corporation Tax late filing penalties
• GOV.UK: Company Tax Returns, making changes
• GOV.UK: Accounting periods for Corporation Tax
• HMRC: Corporation Tax, trading and non-trading
• GOV.UK: Corporation Tax rates and reliefs
• HMRC: Company Tax Return (CT600) and supplementary pages
• HMRC: Businesses XBRL guide
• HMRC: COTAX Manual COM130010
• HMRC: Closure of the service to file your company accounts and tax return
• GOV.UK: Dormant for Corporation Tax
• Companies House: Life of a company, part 1 accounts
• Companies House: Late filing penalties

Fees, deadlines and rules on this page were last checked on 27 September 2026.