CT600 guide

What a Company Tax Return includes

A Company Tax Return is more than the CT600 form. It includes the CT600, any relevant supplementary pages, the company accounts and the tax computations. Together, those documents explain the taxable profit or loss and the Corporation Tax position for the accounting period.

The annual accounts and Company Tax Return have different recipients. Companies House receives the annual accounts; HMRC receives the Company Tax Return. The figures are connected, but paying Corporation Tax or filing accounts does not remove the separate return obligation when HMRC has issued a notice to deliver one. See HMRC's Company Tax Return obligations for the current return requirements.

How do you file a CT600 in 2026?

The former joint HMRC and Companies House online filing service closed on 31 March 2026. From 1 April 2026, an ordinary Company Tax Return is filed using commercial software. The software needs to support the parts of the return the company needs, including the CT600, tax computation and accounts where applicable.

HMRC says paper filing is only available in limited circumstances, including a reasonable excuse for not filing online or filing in Welsh. Do not leave software or agent arrangements until the filing deadline. Check GOV.UK's post-closure filing guidance and the current recognised software list before choosing a route.

What should be ready before the return is prepared?

• reconciled accounting records and the annual accounts for the period
• a tax computation that reconciles accounting profit to taxable profit
• support for capital allowances, losses, reliefs, donations or any other material claim
• the relevant company, accounting-period and associated-company information
• evidence for director transactions, loans or other supplementary-page entries where they apply

The exact supplementary pages depend on the company's facts. The safe sequence is to establish the records and computations first, then use the filing software and current HMRC guidance to determine what the return requires. A generic box-by-box guide can become inaccurate when the form or a company's circumstances change.

Which deadline comes first?

For most companies, Corporation Tax is due nine months and one day after the end of the Corporation Tax accounting period. The Company Tax Return is usually due twelve months after the same period ends. The payment date therefore normally arrives before the return deadline.

The accounting period is normally the same as the period covered by the annual accounts, but it can differ, especially in a first year or when dates change. Use the actual period in HMRC's notice and keep the Companies House accounts deadline separate. GOV.UK's limited-company accounts and tax-return overview sets out the usual dates.

What are the common avoidable errors?

• treating the statutory accounts, Corporation Tax payment and CT600 as one deadline
• submitting a return from incomplete bank, payroll, sales, purchase or director-loan records
• treating a capital cost or a personal cost as an ordinary trading deduction
• claiming a relief without the records and conditions that support it
• making an amendment late instead of correcting an error promptly

A filed return is a formal declaration that it is correct and complete to the best of the declarant's knowledge and belief. HMRC's acknowledgement only confirms receipt; it does not confirm that HMRC agrees with the figures. If an error is found after filing, tell HMRC promptly. Changes are usually made within 12 months of the filing deadline, subject to the rules for the company and amendment.

What records should the company keep?

Keep the accounts, computations and records that support the return together. This includes invoices, contracts, bank records, payroll information, receipts and documents that support reliefs or adjustments. GOV.UK says companies normally keep records for six years from the end of the financial year they relate to, and longer in specified situations such as a compliance check, late return or longer-lived asset.

If the company has a loss or no Corporation Tax to pay, do not assume that no return is needed. If HMRC has issued a notice to deliver a Company Tax Return, the return remains an obligation. The company should check the notice, its accounting period and its records before deciding how to proceed.