What is a CT600? 2026/27 UK Company Tax Return Guide

To manage corporate compliance effectively, tax professionals and company directors must understand the strict statutory framework governing the UK company tax return.

To answer the core question—what is a CT600—it is the prescribed form used to deliver a company tax return to HM Revenue & Customs (HMRC). Under Schedule 18 to the Finance Act 1998, an officer of Revenue and Customs may by notice require a company to deliver this return.  The return must contain specific information, accounts, statements, and reports relevant to calculating the company’s corporation tax liability.

Below is an exhaustive breakdown of how the CT600 operates, the supplementary pages you must attach, and the critical procedural and penalty changes taking effect in the 2026/27 tax year.

The Anatomy of a CT600 and Supplementary Pages

You must generally submit the CT600 electronically, alongside full statutory accounts and tax computations in iXBRL format. Validation rules ensure that all mandatory parts of the electronic CT600 and any required supplementary pages are completed without arithmetical errors.

Depending on the specific transactions a company undertakes during its accounting period, you must complete mandatory supplementary pages.  The most common schedules include:

• CT600A (Close company loans): You must include this if a close company makes loans or advance arrangements conferring benefits on participators.
• CT600B (Controlled foreign companies): Required if a UK company holds a 25% interest in a foreign company controlled from the UK.
• CT600C (Group and consortium relief): You must submit this when making claims or surrenders under the group and consortium relief provisions.
• CT600E (Charities and CASCs): Required for charities and Community Amateur Sports Clubs claiming tax exemptions.
• CT600L (Research and Development): Dedicated schedule for capturing R&D tax credit claims.

Statutory Deadlines: Filing vs Payment

You must strictly separate the deadline to file the CT600 from the deadline to pay the tax owed.

Under paragraph 14 of Schedule 18 to the Finance Act 1998, the statutory filing date for a company tax return is generally 12 months from the end of the accounting period for which the return is made.

Conversely, the standard corporation tax payment deadline falls significantly earlier. For most companies, the tax is due and payable nine months and one day from the end of the accounting period.  Therefore, you must finalise the tax calculations and pay the liability three months before you are legally required to file the CT600.

Mandatory 2026/27 Procedural Changes

The 2026/27 tax year introduces two significant operational shifts that directly impact how and when you file a CT600.

The Closure of HMRC’s CATO Service (31 March 2026)

Historically, unrepresented companies with straightforward affairs could use HMRC’s free online Corporation Tax service (often referred to as CATO) to generate and submit their CT600. However, HMRC has confirmed that this service will permanently close on 31 March 2026.  After this date, all companies excluded from the remaining narrow exceptions must procure and use commercial tax software to file their returns.

Finance Act 2026 Late Filing Penalty Increases (1 April 2026)

To restore the real-terms value of financial deterrents against inflation, the government is significantly increasing the flat-rate penalties for failing to file a CT600 on time.

For returns where the statutory filing date falls on or after 1 April 2026, the penalties double:

• Initial failure: The penalty for delivering a return up to three months late increases from £100 to £200.
• More than 3 months late: The penalty increases from £200 to £400.
• Repeated failures: If a company fails to file on time for a third successive accounting period, the escalated penalties jump to £1,000 (initial) and £2,000 (more than 3 months late).

“The penalties are therefore being increased to restore their original real terms value… This measure will have effect for returns for which the filing date is on or after 1 April 2026.”

Summary Table: CT600 Requirements and 2026/27 Rules

Next steps for research: Audit current client software provisions to ensure all unrepresented clients currently relying on the HMRC CATO service are migrated to an approved commercial software vendor before the 31 March 2026 cutoff, and review portfolios for clients at risk of the new £2,000 successive failure penalty.