Documents Needed for a Company Tax Return: 2026/27

When preparing a UK corporation tax submission, advisors and companies must ensure absolute compliance with statutory filing requirements. Under Schedule 18 to the Finance Act 1998 (FA 1998), HMRC holds the legal authority to require a company tax return containing specific information, accounts, statements, and reports relevant to the company’s tax liability.

To answer the primary question—what documents do I need to file my company tax return—you must submit three core components: a completed Form CT600 (including relevant supplementary pages), full statutory accounts in iXBRL format, and tax computations in iXBRL format.

Below is an exhaustive breakdown of the required documents, formatting rules, and critical procedural changes taking effect for the 2026/27 tax year.

The Core Return: Form CT600 and Supplementary Pages

The foundational document of any company tax return is the Form CT600, which must include a self-assessment of the amount of tax payable by the company for that period.  The return must include a declaration by the person making it that it is correct and complete to the best of their knowledge.

Depending on the company’s activities during the accounting period, the CT600 must be accompanied by mandatory supplementary pages.  HMRC validation rules ensure that these electronic pages are completed where relevant circumstances apply.  Common supplementary pages include:

• CT600A: Required for close companies that have made loans or advance arrangements conferring benefits on participators or their associates.
• CT600B: Required if the company held a 25% interest in a controlled foreign company (CFC) or claims foreign permanent establishment exemptions.
• CT600C: Required for companies claiming or surrendering group or consortium relief.
• CT600E: Required for charities or Community Amateur Sports Clubs (CASCs) claiming tax exemptions.

Full Statutory Accounts (iXBRL Format)

You must file the company’s full statutory accounts, which must include the reports of the directors and the auditors as required by the Companies Act 2006.

For most companies, these accounts must be delivered online in iXBRL (Inline Extensible Business Reporting Language) format.  This requirement applies if the accounts are prepared using an accounting standard supported by an HMRC-accepted XBRL taxonomy, currently encompassing:

• EU adopted IFRS
• UK GAAP (FRS 101, FRS 102, and FRS 105)
• US GAAP

“If the accounts are prepared to an accounting standard not supported by HMRC, they do not have to be filed in iXBRL but must be submitted as a PDF attachment to the online return.”

If a company is permitted to attach PDF accounts under the unsupported standard exception, they must still include a UK GAAP/IFRS profit and loss account for their UK property business within the corporation tax computation in iXBRL format.  Unincorporated charities, clubs, societies, or non-resident companies without a permanent UK establishment may submit accounts in either iXBRL or PDF format.

Tax Computations (iXBRL Format)

The third mandatory document is the tax computation. This must clearly demonstrate how the figures in the CT600 return have been derived from the figures in the statutory accounts.

These computations must also be submitted in iXBRL format.  Where a company has a long period of account, separate computations must be provided for each accounting period falling within that overarching period of account.

2026/27 Procedural Changes: HMRC Service Closure & Penalty Increases

The 2026/27 tax year introduces two major compliance shifts regarding how and when you file these documents.

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

Historically, unrepresented companies with straightforward affairs could use HMRC’s free Company Accounts and Tax Online (CATO) service to generate iXBRL accounts and computations.  This service is officially closing on 31 March 2026.  After this date, all companies must use approved third-party commercial software that produces a return meeting HMRC specifications, or engage a tax agent to file on their behalf.

Finance Act 2026 Penalty Increases (1 April 2026)

The deadline to deliver these documents is generally 12 months from the end of the period for which the return is made.  However, the Finance Act 2026 has significantly increased the flat-rate penalties for failing to meet this deadline.

For failures to deliver a company tax return where the filing date falls on or after 1 April 2026:

• The initial flat-rate penalty increases from £100 to £200.
• If the return is more than three months late, the penalty increases from £200 to £400.
• For consecutive late filings, the escalated penalties have increased from £500/£1000 to £1000/£2000.

Summary Table: Filing Requirements and Changes

Next steps for research: Review the specific commercial software vendors approved by HMRC to replace the CATO service before the 31 March 2026 deadline, and audit current client portfolios for companies with filing dates on or after 1 April 2026 to mitigate exposure to the increased FA 2026 penalties.