Thousands of women who were underpaid the state pension are set to get payouts, with some women estimated to be owed more than £30,000.
Over the next few years it's estimated that around £3 billion could be payed out to women who've been short changed.
An administration error identified in March 2020 suggested some people had been underpaid, according to Office for Budget Responsibility (OBR) documents.
The underpayment affected married women whose husbands reached pensionable age before 2008 and who were unknowingly entitled to an "enhanced pension" that would have boosted their payments by up to 60%.
Department for Work and Pensions (DWP) investigations between May and December 2020 uncovered a systematic underpayment of state pensions, meaning tens of thousands of married, divorced and widowed people may have been underpaid since 2008.
A repayment programme started in January 2021.
The report said: "Our forecast reflects an initial estimate that it will cost around £3 billion over the six years to 2025-26 to address these underpayments."
It warned that the costing "is subject to a high degree of uncertainty as the true extent of the underpayment is not yet established".
Sir Steve Webb, a former pensions minister who is now a partner at pensions consultants LCP (Lane Clark & Peacock), which has previously heard about some refunds amounting to more than £30,000, said: "This figure is truly mind-numbing. When I first looked into this issue a year ago I had no idea it would explode into such a huge issue.
Keep up to date with local news in your area by adding your postcode below
"Repayments of £3 billion over the next five years could imply huge numbers of women have been short-changed, potentially for a decade or more.
"The Government needs to devote serious resources to getting these repayments out quickly as these women have waited long enough."
