By Nick Triggle
BBC News health reporter
The government is unveiling reforms of incapacity benefit this week in a bid to reduce the number of claimants.
To date, doctors have played a key role in helping determine who gets the benefit.
How does the system work, and do doctors need more help administering it?
GPs are asked to detail claimants' symptoms
There is an old saying in the medical profession that there are only three reasons to lie to a doctor.
One is to get a larger insurance pay-out, the second is to get you moved from your prison cell and the third is to get out of work.
To become eligible for incapacity benefit, most claimants have to undergo an assessment by a doctor.
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.
"Yes, you can ask all the questions you want but in the end you have to go on what the patient says in the assessment.
"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.
He said the average consultation last little over three-quarters of an hour.
"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."
However, contrary to most assumptions, the incapacity benefit medical assessments are not done by their own GPs.
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.
The service is contracted out by the government to a firm called Atos Origin under a £500m seven-year deal.
The company employs 250 doctors full-time and another 1,000 - many of them GPs - part-time.
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.
"Some cases are straightforward and take less time, while the more complex cases can run for longer. It is not a hard and fast rule."
More than 2.6m people are claiming incapacity benefit in the UK
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.
If government officials do not feel the Med Four form gives enough information they can request more.
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."
But if early indications are anything to go by, GPs may becoming more involved in the process.
Government officials have suggested doctors could be offered cash incentives and work alongside employment advisers to reduce the numbers out of work because on long-term sickness.
Both schemes were dismissed by Dr Holden. He said it was obscene to try to influence doctors' decision-making with money, and said there was not room for employment advisers in most practices.