Dr Peter Holden, a Derbyshire GP who used to carry out the examinations, said: "If someone says they are unable to work for a bad back they can come in and make out they can't move.

"I have known occasions when claimants would say they are in a lot of pain and move around gingerly, only to walk freely across the car park and when you attempt to put that on the assessment you can't."

Part of the cause of the spiralling number of claimants, said Dr Holden, a member of the British Medical Association's GPs committee, is that doctors are not given enough time to carry out assessments.

"If someone has a complex mental health problem, it is just not possible to tease that out in an hour. You need much more time. The government is getting the service on the cheap."

Under the current system, people are signed off from work by their family doctor for up to six months, during which time they are entitled to statutory sick pay.

Towards the end of that period, if the claimant cannot work the doctor fills in a form called Med Four which basically describes the person's symptoms.

That is then passed to the Department of Work and Pensions which can request more information, but has its own team of doctors to carry out an assessment.

Even then doctors are not allowed to give a judgement on whether a person should be entitled to the benefit, but instead are asked to describe how their illness or disability impairs their functional capability.

Although there is no target time for consultations, Atos acknowledges that to be "efficient" the average assessment should only take about 45 minutes.

But chief medical officer Carol Hudson said it is enough time to determine a claimant's condition. "Our doctors are trained to carry out these assessments and we are confident they are getting it right.

While GPs are kept out of the assessment process, Dr Holden admitted that they can get bogged down with correspondence with the Department of Work and Pensions.

He said: "You can find yourself filling out lots of forms, but on the whole it is not that burdensome. I think the real problem lies with the assessment part."

However, others are less sure. Joanna Clason, who last year authored a report on incapacity benefit for the NHS Confederation, which represents health service managers, said family doctors should be taken out of the process altogether.

"Whatever, time it takes, it is time the GP could be spending with patients. We believe the responsibility should be taken away from them and handed to an independent panel of health professionals."

This is cache, read story here