Up to 8.7million pensioners may have paid too much tax last year.
HMRC pocketed millions more than it was meant to due to the error, which saw pensioners pay about £5 extra each.
HMRC knows the error went undetected for about 10 months, prompting fears it could have been going on for years.
According to the Sunday Times, the issue was flagged in August last year by Tory MP Richard Holden, but was not officially reported to the Department for Work and Pensions (DWP) until October.
HMRC hopes to resolve the issue this summer. The department is now looking to learn how many people are affected, but it is not issuing automatic refunds at present.
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A spokesperson said: ‘We apologise to those affected by this calculation error and are working to fix the issue, although the impact is small with the difference in tax owed being around £5 in most cases.’
How did this happen?
The state pension – which rises annually in April in line with the highest figure of inflation, average earnings or 2.5% – is paid gross, but recipients still have to pay income tax on it.
According to HMRC guidance, the amount of tax a pensioner owes should be calculated based on 51 weeks of the current tax year’s pension rate and one week of the previous year, to account for the time between the start of the first tax year on April 5 and the first payment after that.
But HMRC calculates income using 52 weeks of state pension payments at the higher rate using DWP data.
This means that after the new state pension for 2025/26 of £230.25 a week was instated – up from £221.20 in 2024/25 – income would have been recorded as £9.05 a week higher than it actually was.
Records show those affected paid, on average, an extra £5 in tax.
Critics have urged the department to reveal exactly how long the problem will take to fix.
Sir Mel Stride, the shadow chancellor, said: ‘If HMRC have been charging millions of pensioners too much tax then questions need to be answered, and the matter must be urgently put right.
‘Ministers need to ascertain what has happened and what action is being taken to ensure these sorts of errors do not happen again.’
Dan Tomlinson, the minister responsible for HMRC, previously said ‘most pensioners pay the right amount of tax in real time.’
But he added: ‘HMRC has become aware that for a subset of individuals in receipt of the state pension, a calculation error means that their tax is calculated based on 52 weeks at the new rate.
‘The difference in tax owed is approximately £5. Affected individuals can call HMRC to amend any incorrect figures of state pension.’
Those who have been affected should contact HMRC for a refund. They will continue to pay tax as normal.
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