OTTAWA - Prime Minister Stephen Harper's legal team plans to ask a judge to summon B.C. author Tom Zytaruk to Ottawa to testify in Harper's defamation suit against the Liberal party over bribery allegations in the Chuck Cadman affair.

The Vancouver lawyer for Zytaruk and a Toronto lawyer representing the Liberal party confirmed Thursday that Harper's lawyer has signalled he wants Zytaruk to appear personally to attest to the authenticity of a recorded interview of Harper that is at the centre of the lawsuit.

If the motion compelling Zytaruk to appear succeeds, he would testify in September, when the first hearing in the $3.5-million lawsuit has been scheduled.

In the audio recording of Zytaruk's 2005 interview with Harper, then opposition leader, Harper is heard saying he was aware Conservative party organizers spoke to the late MP Cadman about "financial considerations" before a Commons vote earlier that year that could have led to a snap election.

Cadman, a former Conservative who was an Independent MP at the time and had terminal cancer, supported the Liberal government in the confidence vote to prevent the election.

Harper tells Zytaruk in the recording it was his understanding the organizers, Harper's top political operative Doug Finley and campaign director Tom Flanagan, approached Cadman with an offer only to replace financial considerations he might lose "due to an election."

Zytaruk wrote in a biography of Cadman, who subsequently died, that his widow Dona said the Conservatives had offered Cadman a $1-million insurance policy if he helped to defeat the Liberals and force an election the Conservatives were expecting to win because of controversy over the Liberal sponsorship scandal.

Harper launched his lawsuit following turmoil over the allegations last year, and last month added a further $1 million in damages for "misappropriation of personality" on grounds Zytaruk's tape had been doctored to distort what Harper said to the author.

It is the integrity and authenticity of that tape that could be at the centre of arguments when the first hearings in the lawsuit take place Sept. 22-23, said Chris Paliare, the Liberal party's lawyer.

"That's my understanding," Paliare said, adding he understands Harper's lawyer, Richard Dearden, plans to file motions within a week asking an Ontario Superior Court judge to order a summons compelling Zytaruk to appear.

Personal testimony in a libel suit would be unusual, even more so if the witness is from another province, Paliare and Zytaruk's counsel, Vancouver lawyer Barry Gibson, both told The Canadian Press.

Gibson confirmed Zytaruk is aware of the possibility, and also confirmed Zytaruk has refused to furnish Dearden with an original copy of the recording for verification by audio and computer experts.

"For obvious reasons, Mr. Zytaruk doesn't want to give up the originals, especially to Dearden," Gibson said in an interview.

Though it has been unable to obtain an original copy of the tape, the Conservative party has nonetheless had experts examine a copy Zytaruk provided and says the experts determined the original recording had been altered.

Dearden is also expected to file an earlier motion in a British Columbia court to seek a hearing in B.C., where Zytaruk would also appear with his original recording of Harper to initially verify the recording is authentic and prepare the ground for the Ottawa hearing.

The hearing has been scheduled to hear Harper's application for a court injunction preventing the Liberal party from using the recording, which the Liberals say in their statement of defence was widely available on the Internet when they downloaded it.

Dearden said he could not comment on the case, and had been instructed to refer journalists to the prime minister's communications office.