Medical assessments of benefit applicants at Atos Healthcare were designed to incorrectly assess claimants as being fit for work, one of the company's former senior doctors has claimed.

Greg Wood, a GP who worked at the company as a senior adviser on mental health issues, said claimants were not assessed in an "even-handed way", that evidence for claims was never put forward by the company for doctors to use, and that medical staff were told to change reports if they were too favourable to claimants.

The doctor claimed he resigned in disgust at what was going on, saying that many doctors he had spoken to shared his concerns. "I think the Department for Work and Pensions is the real culprit here. It's the government training that makes Atos assessors do this."

The work capability assessment is used for the government's employment and support allowance, a benefit paid to those too sick to work. Wood said that the assessors were trained in such a way that they expected claimants to score too few points to qualify for ESA, and to award points "begrudgingly".

The attitude drilled into assessors "leans towards finding reasons not to award points", he claimed. The result was a bias against the disabled, he said.

Last year the British Medical Association called for the tests to be scrapped to prevent harm to the most vulnerable people in society. Wood said that although his contract with Atos had a confidentiality clause, he was breaking it in the "public interest". He told the Guardian: "In my experience [Atos assessors] are not free to make independent recommendations, important evidence is frequently missing or never sought in the first place, medical knowledge is twisted and points are often wrongly withheld through the use of an erroneously high standard of proof."

He said if Atos assessors "show deviation from the official line they are instructed to change the report. In about a quarter of assessments important documentary evidence such as the claimant's own GP assessment is missing but the assessments go ahead regardless."

Although work capability tests were introduced by Labour in 2008, the coalition has rapidly expanded their use.

Atos – which last year processed almost 20,000 incapacity benefit claimants a week – has faced criticism after it emerged that a third (37%) of decisions appealed against were successfully overturned.

There have also been repeated claims that people with terminal cancer have been denied benefits as a result of Atos assessments and that the company sets out to strip people of benefits by making the tests arduous and degrading.

Labour MP Tom Greatrex, who has campaigned for reforms to Atos and the fitness to work test, said: "These are very serious and shocking allegations which must be urgently looked at. I have written to the prime minister today asking him to personally order an investigation.

"Those who can work should be helped into employment through effective back-to-work schemes, and those who can't through illness or disability should be supported. It's about helping people, not hounding them. Based on the evidence of Dr Wood this system is failing us all in the worst possible way."

In a statement Atos said it "completely refuted allegations made by Dr Wood that Atos Healthcare acts inappropriately or unethically. We never ask healthcare professionals to make any changes to a report unless there are specific clinical quality issues identified within it. A report that may need to be revised could, for example, be one where there is insufficient justification recorded to support the advice given.

"We do not deviate from government guidelines in our training. We do not have targets for getting people on or off benefits. We are a professional and ethical organisation which has carried out this work on behalf of the department for over a decade.

"Atos Healthcare conducts its business based on a code of ethics and a strong legal compliance culture. Like any other organisation we are, quite properly, subject to scrutiny. However, Atos Healthcare will take appropriate legal steps to defend itself and its employees where false and damaging allegations are made."

The Department for Work and Pensions said the claims had been "well-aired before". "A decision on whether someone is well enough to work is taken after a thorough assessment and consideration of all the supporting medical evidence provided by the claimant."

"There are absolutely no targets regarding how many people should be found fit for work and since 2010 we have considerably improved the work capability assessment (WCA) process. The percentage of people entitled to employment and support allowance is now at its highest level with over half of people completing a WCA eligible for the benefit."