The Department for Work and Pensions (DWP) estimates it has underpaid 134,000 pensioners, mostly women, over £1 billion of their state pension entitlement, with some errors dating as far back as 1985.
In January 2021, DWP started an exercise to correct the errors - the ninth such exercise since 2018, the Public Accounts Committee (PAC) said.
The errors mostly affect widows, divorcees and women who rely on their husband's pension contributions for some of their pension.