The expert in charge of the controversial ‘back to work’ tests for incapacity benefit claimants is to quit after warning that the system needs to be made more ‘fair and humane’.

Professor Malcolm Harrington will step down as a government adviser later this year after carrying out a third review of the testing system that assesses whether the long-term sick and disabled are fit to look for work.

He made a series of controversial recommendations which have actually reduced the number of people considered to be fit for employment.

The Department for Work and Pensions last night rejected suggestions that Professor Harrington had been forced out, saying that he had only ever been due to conduct three reviews.

But the occupational therapist told the BBC’s Panorama programme that he had been told a ‘fresh pair of eyes’ was needed. He said: ‘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”.

Controversial work capability assessments were piloted by Labour in 2008 and have been rolled out across the country by the Coalition. More than two million recipients of employment and support allowance – the successor to incapacity benefit – are now having their health assessed by private contractor Atos.

Ministers argue it is vital that people able to work are moved off benefits and back into employment. They accuse the last government of abandoning people to a life on benefits.

The DWP released figures at the weekend showing that thousands of people had been on incapacity benefit for more than a decade because of drink and drug problems. Some have been signed off for years with complaints such as acne, obesity and coughs.

But critics claim the tests are heavily skewed towards forcing people back into work, causing huge distress for some who are genuinely ill.

Professor Harrington was called in to review the tests after thousands of disabled people successfully challenged their test results.

He recommended a series of changes which have cut the number of people judged to be fit to work. He told the BBC that while ministers had accepted his proposals there was still a long way to go.

‘I think people are being treated more like human beings now, but it is still difficult to go through,’ he said.

The DWP last night insisted that Professor Harrington had always been due to leave at this point. A spokesman said: ‘It was made clear at the start that Professor Harrington would undertake three reviews.’

Ministers are furious about a separate investigation into the testing system by Channel 4’s Dispatches programme which was due to be broadcast last night.

Secret filming suggested pressure was applied to doctors by Atos to find people fit for work. Doctors were warned that if they judged more than 12 per cent of candidates to be in need of long-term sickness benefits they were likely to have their work reviewed.

Ministers are considering a complaint to Ofcom over the programme after it emerged that Dr Steve Bick, who conducted the undercover filming, is a Labour activist who stood at the last election.