Source BBC: Assessors wrongly calculated the income of thousands of people during the process that resulted in people moving from incapacity benefit and severe disability allowance onto ESA.
Tens of thousands of people on sickness benefits will receive backdated payments averaging £5,000 following government errors.
The Department for Work and Pensions (DWP) has revealed it is paying more than £1.5bn due to the mistakes.
Some people have already received payouts of more than £10,000.
The mistaken calculations were made when people were moved on to the main sickness benefit, the Employment and Support Allowance (ESA).
The errors were first revealed by BBC News in November, but the scale of the underpayments has now emerged. Read on.