The tool helps to work out transition profit for the 2023 to 2024 tax year for anyone affected by the new tax year basis period reform rules, except partnerships, if the accounting year end does not match the tax year. In other word, it is not on or between 31 March and 5 April.

'Around 280,000 sole traders along with around 250,000 partners in partnerships and LLPs will be affected by the changes being introduced by basis period reform, also known as tax year basis reporting,' HMRC said. 'There is guidance available for all those affected on gov.uk. Limited companies are unaffected by the changes.'