Thousands of people may have been underpaid benefits due to a state pension error.
Around £1billion may be owed to people - mainly women in their 60s and 70s - after an issue affecting stay-at-home mums who made a claim for Child Benefit before May 2000.
HM Revenue & Customs (HMRC) is set to send out letters to thousands, informing them they may be owed additional money.