Prime Minister Stephen Harper didn't show any sign Saturday of backing off a defamation lawsuit against the Liberal party despite a finding that a key audio recording was not altered as he claimed.

"The Liberal party's got a big problem in court - that's all I can say," Harper told reporters in London, Ont.

Harper launched a lawsuit against the Liberals during a raging controversy this year over allegations in a book by B.C. author Tom Zytaruk that the Conservatives offered the late independent MP Chuck Cadman a $1-million life insurance policy in 2005 in exchange for his help in defeating the minority Liberal government of the day.

Harper denied he told Zytaruk he was unaware of the "details" of the insurance policy offer. He insisted that he only confirmed the party had offered Cadman "financial considerations" in return for rejoining the Tories and voting against the Liberals in a Commons confidence vote.

At the centre of the controversy is a taped interview that Zytaruk conducted with Harper after Cadman died of cancer in 2005.

The prime minister maintains Zytaruk doctored the tape.

However, a former FBI agent that Harper's side brought in to examine the tape has concluded the key part of the recording was not altered.

But the overall recording was still edited, Harper said Saturday.

"Experts have differences in nuance. But all these reports show exactly what we're saying: that this tape has been edited, and the conversation is incomplete," he said.

Zytaruk maintained Saturday that he did nothing to the tape.

"What I have on the tape there is the interview in the entirety," Zytaruk said in a phone interview. "It doesn't really bother me either way personally what audio analysts say about it because I know what the truth is."

Harper's decision to stand by his legal action didn't appear to cause any concern for Liberal Leader Stephane Dion.

"The main expert of the Conservative party confirmed that the tape has not been doctored. That means Mr. Harper has a lot to explain," Dion said Saturday.

NDP Leader Jack Layton said the prime minister's response makes him question the confidence he has in Harper's ability to lead the country.

"I think all Canadians should be concerned about it," Layton said. "I think it simply just gives us another reason to say we can't have confidence in Mr. Harper to do the right thing.

"He hasn't done it on the economy or with the banks and he's not doing it with the tapes either," Layton added.

Cadman's widow, Dona, is the Conservative party's candidate in Surrey North, B.C., her husband's former seat. She could not be reached from comment.

Liberal Marc Muhammad is fighting Dona Cadman for the seat.

"Most people in this riding today feel that they have got the truth," he said when asked about the tape finding.

Former FBI agent Bruce Koenig, the sound expert Harper hired to prove his allegations, submitted a report dated Friday to Harper's lawyer, which also had to be disclosed to Liberal lawyer Chris Paliare.

In the report, Koenig concluded that the first part of Zytaruk's interview with Harper, which contains the key portions that the prime minister has contested, was intact.

The second part, beginning roughly one minute and 41 seconds into the tape, was a new recording that was made over the final part of the original recording, he said. But the first crucial minute and 41 seconds of the tape had not been altered.

Koenig reported that the tape "contains neither physical nor electronic splices, edits or alterations, except for the over-recording start that erased and replaced the end of the first part of the designated interview."

During a sworn cross-examination conducted by a Liberal party lawyer in August, Harper testified that he authorized his campaign manager, Doug Finley, and former adviser Tom Flanagan, to approach Cadman only with an offer of financial help should Cadman vote against the Liberals and then run for the Conservatives in the election that would have been called.

Koenig's analysis was filed in Ontario Superior Court on Friday by lawyers for the Liberal party, despite attempts by Harper's lawyer to keep the opinion out of the court file until at least next week.

That part of the recording includes Zytaruk's question to Harper on whether he knew anything about a $1-million insurance policy that unidentified Conservatives had allegedly offered to Cadman in return for his support in Parliament against the Liberals.

"I don't know the details, I know that, um, there were discussions, um, but this is not for publication?" Harper replies on the tape.

Harper sued the Liberal party last March after it posted headlines on its website about the Cadman affair.

In a statement of defence filed this year, the Liberals contended that Harper's lawsuit is an infringement of free political comment and violates the Charter of Rights as well as sections of the Canadian constitution.