CT600: what is a Corporation Tax return?

The form identifies the company and accounting period, then reports taxable trading profit, chargeable gains, losses, reliefs, loans to participators, distributions and tax due. Supplementary pages cover matters such as group relief and restitution tax.

The return package normally includes statutory accounts tagged in iXBRL and a detailed tax computation showing every adjustment from accounting profit to taxable total profits.

A company must file when HMRC gives it a notice to deliver a Company Tax Return. That includes companies with no tax to pay and loss-making companies. A dormant company should tell HMRC it is dormant so HMRC can withdraw future filing notices where appropriate.

Directors are responsible for accuracy even when an accountant or tax adviser prepares and submits the return as agent.

The filing deadline is normally 12 months after the accounting period ends. Corporation Tax itself is normally payable nine months and one day after that date, so payment comes first.

A first set of accounts longer than 12 months often requires two CT600 returns because one Corporation Tax accounting period cannot exceed 12 months.

For returns due from 1 April 2026, a late CT600 normally triggers £200 immediately and another £200 after three months. At six months HMRC can estimate the bill and charge 10% of unpaid tax, with another 10% after 12 months. Three consecutive late returns can increase the fixed penalties to £1,000 each.

These penalties are separate from Companies House late accounts penalties and from interest on late tax payment. Earlier return deadlines can fall under the previous £100 and £500 fixed amounts, so confirm the regime for the return concerned.

Frequent errors include deducting depreciation instead of claiming capital allowances, failing to add back client entertaining, omitting associated companies, forgetting a section 455 charge, carrying losses incorrectly and filing figures that do not match the accounts.

Rates and deadlines are reviewed for 2026/27, but HMRC interest rates and individual circumstances can change the result. Check the linked official guidance or ask us before acting.