David Cameron's official spokesman sidestepped repeated questions from reporters whether the Prime Minister was "confident" that Universal Credit would be delivered on time and on budget.
He said: "The department is working to the 2017 timetable. That is the timetable that the department continues to work to."
Pressed on whether it was right for ministers, rather than senior officials, to take responsibility for failings in implementing major policies, the No 10 spokesman said the Prime Minister had previously insisted that "everyone has to take their responsibility."
The chairperson of the Public Accounts Committee (PAC) said she had not been approached by the Work and Pensions Secretary over singling out the departments' chief civil servant in the report revealing the Universal Credit scheme "wasted £140 million."
The Times reported that Mr Duncan Smith and members of his parliamentary team approached Tory members of the PAC to ensure that the department's chief civil servant Robert Devereux was singled out for criticism.
Sidestepping the question over wider Tory lobbying, she told the BBC:
Iain Duncan Smith didn't approach me. Beyond that I can't comment.
She went on to criticise the "fortress culture" at the Department of Work and Pensions, which meant "only good news was reported and problems were denied".
Commons leader Andrew Lansley said there was "no truth" in allegations that Cabinet minister Iain Duncan Smith tried to blame "shocking" failures in his welfare reform programme on his most senior departmental official.
Mr Lansley said he had spoken to Mr Duncan Smith about allegations that he and members of his parliamentary team approached members of the Public Accounts Committee to ensure that his department's chief civil servant was blamed for the "alarmingly weak" handling of Universal Credit reform.
I have talked to Mr Duncan Smith and I can tell the House that there is no need for a statement.
I can tell you and the House now there is no truth in the allegations made about talking to members of the Public Accounts Committee because I talked to Mr Duncan Smith so I can tell the House.
TaxPayers' Alliance political director Jonathan Isaby says the principle of simplifying the benefits system is right but the implementation is "deeply flawed".
He added: "A horrendous amount of taxpayers' money has already been wasted and those responsible for that must be held to account.
"Such a fundamental reform of the welfare system was always going to be a challenge, but the approach of many officials in the Department for Work and Pensions has been little short of shambolic."
Following criticism of the implementation of Universal Credit, a Department of Work and Pensions spokeswoman has said the scheme will "ultimately bring a £38bn economic benefit to society."
She added: "This report doesn't take into account our new leadership team, or our progress on delivery. We have already taken comprehensive action including strengthening governance, supplier management and financial controls...
"We don't recognise the write-off figure quoted by the committee and expect this to be substantially less. The head of Universal Credit, Howard Shiplee, has been clear that there is real potential to use much of the existing IT. We will announce our plans for the next phase of UC delivery shortly."
A scathing review of the Government's flagship Universal Credit scheme found that some of the IT assets that have been delivered for the programme cannot be used and so must be written off.
Initial estimates suggest the write-offs could amount to at least £140 million.
– Public Accounts Committee report
We saw evidence that purchase orders with a total value of £8.7 million were approved by a personal assistant to the Programme Director.
In another case, two purchase orders, one for £22.6 million and one for £1.1 million, were approved by a personal assistant to the Programme Director whose delegated financial authority at the time of approvals was only £10 million.
When the Department made individual payments to suppliers these could not be linked to particular pieces of work that had been delivered.
The Government's flagship benefit reform has been savaged by MPs for "shocking" failures and "alarmingly weak" management that have already wasted at least £140 million.
Secretaries were allowed to authorise purchase orders worth more than £20 million, and in some cases it is unclear what suppliers have been paid for, according to a damning report by the Public Accounts Committee.