Around 45,000 sick and disabled people may be entitled to around £2,500 each but a small number of people could be owed £20,000.
05:57, UK, Wednesday 21 March 2018
Those affected were transferred to the employment and support allowance (ESA) from other benefits after 2011, the National Audit Office said.
NAO chief Sir Amyas Morse said many of the claimants have severely limiting disabilities and illnesses.
He blamed the Department for Work and Pensions for “failing to get a grip on the problem”.
He added: “The department has now committed to fixing this error by April 2019, but not everyone will be repaid all the money they have missed out on.”
A review of 1,000 sample cases found that around 45,000 people may be entitled to around £2,500 each, but 20,000 could be owed around £11,500 each and “a small number of people” could have been underpaid by £20,000.
Around 70,000 people are thought to have been underpaid benefits by an average of £5,000 each, according to the public spending watchdog.