Shadow work and pensions secretary Chris Grayling, who obtained the figures in Parliament, said it was time for change.

He said: “Despite all the grandiose promises and the billions of pounds that have been spent, they’ve delivered virtually no improvement for the most vulnerable in our society.”

Figures released two months ago showed that more than 100,000 drug addicts and alcoholics claim sickness benefits because their “illnesses” make them unfit to work. Incapacity Benefit carries a premium of around £20 a week over ordinary Jobseekers’ Allowance.

And claimants generally do not face irksome demands to participate in schemes such as New Deal.

Attempts to make them prove they really are unfit for any sort of work have prompted outrage from disability rights campaigners.

There have also been accusations that the benefit is too easy to claim.

In April, claimant Joanne Kirman, 24, told the Work and Pensions Department that her bad back made it impossible for her to work. But at the same time she was working at a department office in Blackpool and drawing a salary helping others with their claims.

The Conservatives say they will make all existing and new Incapacity Benefit claimants attend an “in-depth assessment” to evaluate their capabilities. Those deemed fit to work would be put on Jobseekers’ Allowance.

Yesterday’s figures showed the North-west was worst affected, with more than 130,000 long-term claimants.

But the constituency with the biggest proportion of those “on the sick” for a decade was Easington in Durham. It had 42 per cent of its IB claimants receiving payments for more than 10 years.