The error means some taxpayers may have overpaid tax.

The body says HMRC has confirmed that a glitch in the self assessment system omitted Marriage Allowance claims from people’s Self Assessment tax calculations. This has now been fixed.

The Low Incomes Tax Reform Group, an initiative of the CIOT, has urged people who filed their 2018/19 tax return before 21 January using HMRC’s online system to check their calculation to ensure the Marriage Allowance claim was included in order that they do not overpay tax.

The Marriage Allowance is claimable where one spouse or civil partner does not get full use of their personal allowance.

That spouse or civil partner can give up £1,250 of their allowance (for 2019/20; £1,190 for 2018/19) and their spouse or civil partner then obtains a tax reduction of £250 (for 2019/20; £238 for 2018/19). Neither spouse should pay tax at a rate higher than the basic rate (or Scottish intermediate rate, if applicable) before the transfer takes place.

There has been a glitch in HMRC’s Self Assessment system since April 2019 and it was fixed only on 21 January 2020, says the LITRG.

Victoria Todd, head of the LITRG team, said: “There was a glitch in HMRC’s system until this month that meant the recipients of the tax reduction did not see this in their Self Assessment tax calculation for 2018/19 when they used HMRC’s online system to file their tax return.

“HMRC have been working behind the scenes to issue amended calculations to those affected and we urge taxpayers to check their calculations and make sure the tax reduction has been included.

“Some taxpayers may already have made their tax payments, due by 31 January 2020, and have overpaid due to HMRC’s error. Such taxpayers will receive a tax repayment within four weeks and should contact HMRC if they do not.”