A major error in paying disability and sickness benefits to 75,000 people is to be put right - at a potential cost of up to £500 million.
Work and Pensions Secretary David Gauke has said he expects the error to be rectified during the course of 2018-19.
He offered a detailed explanation behind the mistake with Employment and Support Allowance (ESA) in a written statement to Parliament, confirming an estimated 75,000 ESA claimants may have been underpaid.
Gauke said the error occurred because many claimants were not transferred over to the income-related version of ESA they were entitled to, denying them the chance to claim extras such as the enhanced disability premium.
He said officials put in place fresh guidance in 2014 to correct the assessments process, but "did not identify the need to explore the potential impact of the earlier error".
This was picked up in light of fraud and error statistics in May 2016, with ministers not informed until February 2017.