Two private companies have earned more than £500 million over four years conducting fit-for-work assessments of disability benefit claimants.
Data published by the Daily Mirror showed that Atos and Capita were paid a total of £211m for Personal Independence Payment (PIP) assessments in the first 11 months of 2016, £198 million in 2015, £91 million in 2014 and £7 million in 2013 - the year the benefit was launched.
The assessments are carried out when claimants move from the old Disability Living Allowance, which is being replaced by PIP, to determine whether they are entitled to the new benefit.
But the Mirror said that 61% of 90,000 claimants who appealed against a PIP decision at a tribunal in the period up to September 2016 won their case.
Liberal Democrat leader Tim Farron said the figures raised a suspicion that the companies were driven by the profit motive, with the incentive "to get the assessments done, but not necessarily to get the assessments right".
Mr Farron described the firms as "the ugly face of business", while Labour's welfare spokesman Alex Cunningham said: "It is clear that the assessment process continues to fail."
But an Atos Healthcare spokesman told the paper: "The Atos team undertake PIP assessments on behalf of the Department for Work and Pensions and do not take a decision on a person's eligibility for PIP - that decision can only be taken by the DWP.
"Each assessment is carried out under the guidelines delivered by the DWP."
And a DWP spokesman said: "Payments to our providers reflect the amount of work they do for us.
"We introduced PIP to replace the outdated DLA system, and as we invite more people to claim PIP, claims have been steadily increasing quarter on quarter since summer 2015."