Opposition parties hammered the federal Conservatives again Thursday over the proposed purchase of F-35 stealth fighters, accusing the government of misleading Canadians over the true cost of the planes.

But the government stuck to its line that no money has been spent to buy the aircraft and Auditor General Michael Ferguson determined there was no misuse of public money in the process.

"The auditor general made one recommendation and that is the Department of National Defence refine (its) cost estimates for the F-35 and table those in Parliament," Public Works Minister Rona Ambrose said during Question Period Thursday.

"We have frozen the funding and I'm glad the auditor general said today that the government is moving in the right direction," she said.

In his scathing report released this week, Ferguson said the Defence Department manipulated the process, low-balled cost estimates and kept Parliament in the dark to ensure it got the jets it wanted, without competition.

He reiterated his findings again Thursday as he appeared before a Commons public accounts committee.

He said National Defence and Public Works departments accept his recommendation but not his conclusions that due diligence wasn't followed in the procurement process to replace Canada's aging CF-18s.

Interim Liberal leader Bob Rae has demanded Prime Minister Stephen Harper resign, accusing him of lying about the deal.

Rae said escalating problems with the F-35 program have been widely reported and it's just not credible for Harper to claim he didn't know about them.

"They're not prepared to accept any consequences, they're not even prepared to tell the truth," Rae said Thursday in the House of Commons.

Rae demanded that Harper tell Canadians when he first knew the cost estimates for the F-35 jets were higher than his government was stating publicly.

Harper reiterated that his government hasn't purchased the aircraft or signed a contract.

"As we've said, we are responding to the auditor general's recommendations to reassess costs and try a better process in the future to ensure that those cost estimates are more accurate," he said.

NDP Leader Thomas Mulcair said he thinks Defence Minister Peter MacKay should be held responsible - though he stopped short of demanding his resignation.