Nearly £1.7bn is to be paid out to disabled benefit claimants after the government mistakenly underpaid tens of thousands of eligible people.
Thousands who claim Employment and Support Allowance (ESA) are due to be paid on average £5,000 with some people already receiving repayments of more than £10,000.
The Department for Work and Pensions (DWP) has been criticised for the mistakes, which mostly took place between 2011 and 2014 when people were moved from incapacity benefit and severe disability allowance onto ESA.
The DWP released a document revealing that around 180,000 people could be owed arrears payments, with around 105,000 estimated to be repaid during 2018-19 and 75,000 during 2019-20.
According to the DWP, backdated payments totalled £970m while the larger ongoing cost of ESA will amount to more than £700m.
The report said the DWP became aware of some individual errors on cases in 2013 and claims processes were changed in 2014 to prevent future errors.
In 2016, analysis of Fraud and Error National Statistics highlighted the scale of the potential error on historical conversions.
The report said: The [DWP] has prioritised checking the claims of individuals whom we know from our systems to be terminally ill.
“Thereafter we are working through contacting cases identified as most likely to have been underpaid according to our systems. Some of the cases most likely to be in error are also some of the more complex cases.”