The man in charge of reviewing the government's controversial fitness-to-work benefit assessments is to stand down, the BBC's Panorama has learned.
Professor Malcolm Harrington has called for a big overhaul of the process of testing claimants' ability to work to make it more "fair and humane".
He said ministers wanted a "fresh set of eyes" - but denied this was because of changes he had asked for.
Officials said he had only been recruited for a limited period.
More than two million people currently claiming employment and support allowance - formerly known as incapacity benefit - are having their status reviewed as part of a government drive to get more people into work and reduce welfare bills.
The work capability assessments were first introduced on a pilot basis by Labour in 2008 and rolled out across the country by the coalition government.
Critics have said the large number of decisions overturned on appeal show the process is flawed and that tests are too impersonal, not medically rigorous enough and the fluctuating nature of some conditions is not sufficiently taken into account.
Brought in to review the system in 2010, Prof Harrington has recommended major changes - such as putting a single health professional in overall charge of each case rather than relying on responses to questionnaires and computer assessments handled by staff from contractor Atos.
Prof Harrington - an occupational health specialist - told the Panorama programme he would stand down after he produced his third review later this year and someone else would take over.
He made it clear it was the government's decision, but rejected suggestions that he had effectively been sacked.
"They said to me 'you have been doing this for three years and you have come up with a number of recommendations which we are going to implement... we think it would be a good idea if a fresh set of eyes looked at it for the final two years'."
Prof Harrington said there was no political pressure for him to go and since he had not "come up with any bright ideas" in the last few months the time may be right for a change.
"At no stage has anybody... ever said to me we are not going to do that because we cannot be bothered or it is too much trouble. No recommendation has been turned down."
Under the system, claimants are placed in three categories: those deemed able to work straight away, those considered able to do so at some point in the future with the right help - the so-called work-related activity group - and those judged unable to work and needing unconditional support.
Prof Harrington said his recommendations had resulted in a fall in the number of those in the fit-to-work category and a rise in those in the work-related group - the most "difficult" group to assess.
Although he believes his recommendations have improved the system, Prof Harrington said changes were not happening quickly enough and the experience was still "traumatic" for many people.
"I think people are being treated more like human beings now but it is still difficult to go through it."
Those challenging decisions often found themselves in "a state of limbo and increased anxiety" for months, he added, while it was "illogical" that some people were being asked to go for further tests just weeks after they had been found unable to work.
"I would like to think it was dramatically better and my recommendations have done a fantastic job. I am not sure that is true. I think it is better, it is improving but it is still patchy."
While people had been let down by the system, he challenged campaigners who had called for it to be scrapped entirely to come up with an alternative.
"What are you going to put in its place? Tell me why, how you are going to make it better."
Ministers have said the welfare state will continue to support those in "genuine need" but "tough decisions" have to be taken to tackle the deficit.
The Department for Work and Pensions said it had made clear in November that this would be Prof Harrington's final year in the role and his successor should be named before the end of the year.
"It was made clear at the start that Prof Harrington would undertake three reviews," a spokeswoman said. "The department is currently considering its options for the recruitment of Professor Harrington's successor and their terms of reference."
Panorama: Disabled or Faking It? will be shown on Monday at 8.30pm on BBC Two.