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.
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.