MISTAKES in paying out benefits claims could cost up to £500m to put right, the BBC has learned.
The errors identified by the Department for Work and Pensions affect the main sickness benefit, the Employment and Support Allowance (ESA).
The BBC understands that assessors wrongly calculated the income of around 75,000 claimants.
Ministers say that they are aware of the problem and that repayments have begun to be made.
The department, which says it discovered the mistakes last December, is understood to have contacted about 1,000 people so far.
It says it is still trying to understand the scale of the problems with ESA, which is paid to about 2.5 million people, and will contact anyone affected.
Mr Field said people had been ‘wrongly impoverished’ as a result of the errors
Frank Field, chairman of the Commons work and pensions select committee, said the problem was on a scale of “historic proportions”.
He said: “I’m still gobsmacked at the size and the nature and the extent and the coverage of people that have been wrongly impoverished by the department getting it wrong.”
The BBC understands that the errors affected people who applied for ESA between 2011/12 and 2014/15 – claimants after that date are understood to have had their benefit correctly assessed.
On top of money to be paid back, the Treasury will have to pay for the staffing and processing of repayments.