HMRC Corporation Tax period differs from your company year end: what to do

First-year exception. Do not ask HMRC to move the first of two returns for a long Corporation Tax coverage to the company year end. A Corporation Tax accounting period cannot exceed 12 months. Accounts longer than 12 months can still have only one trading return if trading began later. Read how two returns can cover long accounts and why a first CT600 may start later.

1. Differences that are expected

Corporation Tax periods are normally the same as the financial year in the accounts, but HMRC lists exceptions. For example, a company incorporated on 1 March 2024 with first accounts to 31 May 2025, and trading from incorporation, needs a return from 1 March 2024 to 28 February 2025 and another from 1 March to 31 May 2025. The first return correctly ends before the accounts year end.

The first tax period may also start when the company first comes within the charge after incorporation. A tax period can end when trading stops. These differences do not by themselves mean HMRC's records are wrong. If dates are unclear, call HMRC instead of changing them only in filing software.

2. Why later dates can drift

The company may have changed its year end at Companies House without updating HMRC, or HMRC may still show dates recorded when Corporation Tax services were added. A period of dormancy or a restart can also explain why the current dates differ. Ask HMRC to align future accounting periods with the company year end. The difference can affect which returns HMRC expects and their deadlines. HMRC warns that failing to update accounting-period dates after extending a company financial year can lead to a late filing penalty.

3. Call HMRC about a mismatch

1. Check the dates in the HMRC business tax account and on the CT603 notice, alongside the Companies House accounts dates.
2. For a later-year mismatch outside the first-year and stopping-trading cases, call HMRC Corporation Tax enquiries. Have the company's UTR, company number, accounts dates and the dates shown by HMRC ready.
3. Ask HMRC which returns it expects, whether its records or notice need correcting, and to align future accounting periods with the company year end.
4. Use the dates HMRC confirms before filing. Keep its response with the company's records. A request to correct dates does not by itself cancel a notice or extend a deadline.

For a fuller checklist, read what to do if HMRC Corporation Tax dates appear wrong. If you changed the company year end at Companies House, you may need to update the HMRC dates as a separate step.

4. Check before filing

In SimpleCompanyTax#Simple CompanyTax, under What are you filing?, choose HMRC CT600. Enter the Accounts dates. If HMRC has confirmed different tax dates, select My HMRC Corporation Tax period is different from my statutory accounts period and enter the confirmed Corporation Tax period start and Corporation Tax period end. Check the return-count preview and complete the confirmations shown. If the periods match, leave the separate-date option unticked.

Choose Review filing and preview the documents. Check each CT600 period, the full accounts dates and the tax computation. If the tax period ends before the accounts end, the attached profit and loss uses the entered tax-period figures and omits later activity. Check how the entered figures are used.

This is general guidance. HMRC can resolve its records and notices. Your company or its accountant is responsible for its filing dates and figures.

Official sources

• GOV.UK: Corporation Tax accounting periods and checking dates
• GOV.UK: update HMRC after changing the company year end
• GOV.UK: first accounts when trading started later
• HMRC: when an accounting period begins and when it ends