Civil servants demanded Iain Duncan Smith quit yesterday as he limply defended his multimillion-pound write-offs and delays for his controversial universal credit welfare changes.

The Work and Pensions Secretary admitted last week that the 2017 target for the full introduction of the single benefit was a no-hoper - with 700,000 claimants facing a longer wait.

He told the Commons work and pensions select committee yesterday that he was delivering the new system "within the timescale and budget" despite admitting that £40 million had already been written off.

In written evidence to the committee, the Department of Work and Pensions said it was "confident that it has taken, and continues to take, the right remedial steps to address past issues in the universal credit programme."

The committee heard that £151m worth of IT "assets" will be "amortised in a shorter period than anticipated" but the department did not give a definitive figure over how much would be written off in the future.

Mr Duncan Smith denied accusations that millions of taxpayers' money has been wasted, claiming they were "bogus" and "nonsense."

But the public accounts committee found that at least £140m had already been wasted on the project, which was blighted by "alarmingly weak" management.

The cross-party committee branded the scheme "inadequate" and open to fraud in a scathing report.

Ministers claim the initial roll-out of universal credit - which combines six means-tested benefits into one monthly payment - has been a success.

They point to figures suggesting it is encouraging more people to look for jobs in what Mr Duncan Smith described as a "cultural shift."

But critics say the limited implementation of the scheme - which has been plagued by IT and other problems - only involve the least challenging cases.

Shadow work and pensions secretary Rachel Reeves said a "handful" of the promised 1.7m would be switched by 2014-15 and only 400,000 by the following year - less than 10 per cent of the original target.

Ms Reeves said the scheme was in chaos and urged Mr Duncan Smith to hold cross-party talks to rescue it.

Public-sector union PCS declared that Mr Duncan Smith should be the one out of work.

PCS general secretary Mark Serwotka said: “The fact Iain Duncan Smith has clung on for so long is one of the great political mysteries of our time and the question must be asked, how is he still in a job?

“He has presided over not only a disgusting campaign of vilification towards the most vulnerable people in our society but also an absolutely scandalous waste of public money that could have been better spent supporting the sick, disabled and unemployed.”