Cameron backs Universal Credit
Whitehall spending watchdog the National Audit Office said that £34 million had already been wasted in the preparations for Universal Credit, which will integrate several means-tested benefits into a single payment.
The scheme's architect, Work and Pensions Secretary Iain Duncan Smith, was summoned to Parliament to defend the £2.4 billion reform on Thursday, and said he had been forced to intervene after "losing faith" in the civil servants initially entrusted with its implementation.
Mr Cameron acknowledged that Universal Credit was "a very complicated piece of work", and said that he had always taken the view that it should be introduced cautiously over a number of years.
But he insisted that the scheme is "a fundamentally good thing" which will help destroy the poverty traps which keep some unemployed people from finding work.
"You should always look at NAO reports carefully and consider what they say," said the Prime Minister.
"This in my view confirmed something we already knew, which is that Universal Credit is a very complicated piece of work and that's why it needs to be introduced gradually. That was always the plan.
"We never had a big bang plan for Universal Credit, because you are dealing with a complex range of benefits currently paid to people that you are over time going to replace.
"The view has always been pilot schemes and then starting with new benefit claimants and only over time moving to existing benefit claimants, who might be getting Housing Benefit, Working Tax Credit and other benefits, all of which have to be integrated.
"Universal Credit has a very sound intellectual, political and moral basis and that's why it has such widespread support.
"That's why all the (Department for Work and Pensions) permanent secretaries - including Leigh Lewis, who was first running the department when Iain became Secretary of State - became so enthusiastic about it. It's why the Labour Party support it, why so many commentators support it.
"It is a very good idea. It needs to be done, but it needs to be done properly, it needs to be done slowly, it needs to be done gradually. That was always the case, and we should be very flexible about how it's introduced."
He added: "I have always been cautious (about) anything that is as big as Universal Credit and involves large-scale IT programmes.
"But I don't think this report changes the fundamentals, which is that it is a fundamentally good thing, because it does get rid of all those poverty traps - particularly at the low end of the income scale - and it also is a very, very strong anti-poverty measure, because it makes work worthwhile, even when you are only working a few hours a week.
Asked whether it was right for Mr Duncan Smith to blame civil servants for problems in the programme, Mr Cameron said: "Everybody has to take responsibility, but if you go back over what the Government has said about this, we've always said it has to be introduced gradually and it has to take time. I believe that is the case."
© 2013 Press Association