Stephen Harper says he wouldn't have installed former adviser Bruce Carson in his Prime Minister's Office if he had been aware of his past.
The Conservative Leader says he was never told of Mr. Carson's full criminal record.
"Had I known these things, obviously I wouldn't have hired him," the Conservative Leader said after a campaign stop in Ontario's Niagara region.
"I am [just]learning these things as well."
The Canadian Press reported Sunday that Mr. Carson was convicted on five counts of fraud - three more than previously known - and received court-ordered psychiatric treatment before becoming one of Mr. Harper's closest advisers.
His lawyer told the wire service that Mr. Carson disclosed his entire criminal record during a security check that was required to become a senior PMO staffer.
Mr. Harper told reporters he knew some of Mr. Carson's background but not all of it.
He said the Canadian Security Intelligence Service and the RCMP were responsible for vetting staff and the Privy Council Office - the bureaucrats who serve the PMO - made the final decision to grant him security clearance.
"Let me be very clear about the situation: I knew that Mr. Carson had difficulties with the law many, many years ago - some 25 years ago."
The Conservative Leader said that subsequent to this period his former aide had bettered himself. "After that, he had a good employment record. He had worked on Parliament Hill for many decades. ... He had even worked for Parliament itself, for the Library of Parliament. He was well regarded."
He said he doesn't know why he was kept in the dark on the full extent of Mr. Carson's criminal record.
"I don't know why I did not know. Obviously we're going to have to go back and look at our systems. Privy Council Office is going to have to look at its systems to make sure these things get caught."
Mr. Harper pointed out that Mr. Carson is not accused of anything that has to do with his employment in the Prime Minister's Office.
Mr. Carson's lawyer, Patrick McCann, confirmed that his client told him that he revealed all of his past criminal record before he joined the PMO. "That was all done in relation to the security check," Mr. McCann told The Globe and Mail on Monday morning.
The Privy Council Office said it cannot discuss what Mr. Carson revealed when he applied for the job, insisting the Privacy Act prevents such disclosure.
PCO said it is the RCMP that conducts the checks, but a spokesman for the RCMP, Sergeant Julie Gagnon, said that while the RCMP does the checks, it's the PCO that manages the process, and referred all questions back there.
NDP Leader Jack Layton said Mr. Harper cannot plead ignorance.
"I certainly would have thought that anyone working close to the Prime Minister in a key decision-making role would have gone through a significant vetting," Mr. Layton said after a late-afternoon rally in London, Ont.
"And for Mr. Harper to shrug his shoulders and say 'well I'm not sure if really knew anything about it' just doesn't cut it," he said. "How do you allow someone like that into your administration?"
With reports from Campbell Clark and Gloria Galloway