At least half a million people have been wrongly denied incapacity benefit over the last 15 years.
A study by the pressure group Compass found that over 300,000 claimants had their benefits reinstated after appealing to a tribunal.
Another 200,000 people who have been refused incapacity benefit have had it restored at a later point.
The government is planning to expand the use of its “working capability assessment” in an attempt to drive people off benefits.
The tests have been heavily criticised for finding many people with serious illnesses fit for work.