David Cameron has raised doubts over whether his government's flagship welfare reform will be delivered by 2017 - a deadline that has already been delayed once.
The Prime Minister said that universal credit was "immensely complicated" and warned "we should not be religious about timings".
His comments are at odds with those made by the Work and Pensions Secretary just days ago.
Iain Duncan Smith staked his political reputation on a promise that the project, which will see six different benefits rolled into one, will be implemented on budget and within the new timetable.
The minister made the comments after a scathing report by the Government’s financial watchdog.
The National Audit Office concluded that universal credit had been badly managed, overly ambitious and poor value for money.
It warned that £34m invested in a new IT system had already been written off, with the full roll-out already pushed back from 2014 to 2017.
Mr Duncan Smith pointed the finger of blame at civil servants, some of who had been sacked. He told Parliament there was no question about any further delays.
But the Prime Minister was less forthright when he appeared in front of the liaison committee, made up of MPs who chair Parliament’s select committees.
He told Dame Anne Begg, chair of the work and pensions committee, that the key was getting the early stages of implementation right.
"My view is this is a good reform that will make work pay and that is widely supported across politics and other sectors," he said.
"So we need to get it right but we shouldn’t be religious about timings.
"We should be religious about the overall concept of what we are trying to do."
Labour were quick to respond to the comments.
Liam Byrne, shadow welfare secretary, said no one in Westminster believed Mr Duncan Smith's 2017 deadline would be met, adding: "And now it seems the Prime Minister doesn't believe him either."
Mr Byrne had told Parliament that to meet the timetable would mean moving more than 200,000 people a month - equivalent to "a city the size of Derby" - onto the new system.
Citizens Advice warned that the vast majority of people coming into its bureaus had no understanding of what was happening with the reform and how it would affect them.
Citizens Advice chief exec Gillian Guy said: "Constant uncertainty adds to the anxiety people are feeling about the new system.
"More than 23,000 people went to our website for advice on universal credit between April and July this year."