In fact, official figures showed that just 19,700 IB recipients withdrew their claims before facing work capability assessments as part of the transfer to the new employment and support allowance (ESA) between March 2011 and May 2012, said UK Statistics Authority chairman Andrew Dilnot.
Mr Dilnot said that the Tory chairman appeared to have "conflated" figures for IB claimants being moved onto the new benefit with those for new applicants for ESA who had not previously been in receipt of incapacity benefit. Some 878,300 of these claims were closed before undergoing assessment over a three-and-a-half year period from October 2008 to May 2012.
Earlier this month, Work and Pensions Secretary Iain Duncan Smith was rebuked for claiming that figures showed the Government's benefits cap had driven 8,000 people to return to work. In February, Mr Dilnot said David Cameron was wrong to claim in a party political broadcast that state debt was falling. And in December last year, he asked Health Secretary Jeremy Hunt to stop claiming that he had increased the NHS budget in real terms in each of the last two years.
Mr Dilnot's latest comments came in a letter to Labour MP Sheila Gilmore, who raised a complaint over a Conservative press release issued in March this year under the headline: "Nearly 1 million people drop incapacity benefit claim before medical test." Ms Gilmore also complained that press reports of the issue implied that those dropping claims were doing so because they had never really been ill.
Mr Dilnot found: "Having reviewed the article and the relevant figures, we have concluded that these statements appear to conflate official statistics relating to new claimants of the ESA with official statistics on recipients of the incapacity benefit (IB) who are being migrated across to the ESA.
Shadow work and pensions secretary Liam Byrne said: "This is a Government that doesn't like to let the facts get in the way of a good story.
"Grant Shapps may know a thing or two about making things up but it really is outrageous that the Tories have been caught yet again misusing statistics for their own ends. People want a Government that deals with the problems facing Britain, with a plan for getting growth and jobs in our economy, not one that repeatedly misleads the public. Grant Shapps needs to come clean and apologise for trying to pull the wool over people's eyes."
A Conservative spokesman said: "This Government is fixing the welfare system to make work pay. We're capping benefits so that no out-of-work household can claim more than the average working family earns and introducing Universal Credit so it always pays to work. The same old Labour Party have opposed every single measure we've taken to fix welfare."