OTTAWA - A Superior Court judge has ordered a court-supervised analysis of a controversial audio tape at the centre of Prime Minister Stephen Harper's $3.5-million defamation suit against the Liberal party.

Justice Charles Hackland issued a court order Friday compelling author Tom Zytaruk to surrender the tape of an interview he conducted with Harper about Chuck Cadman, the late independent MP.

Harper's lawyer, Richard Dearden, obtained the court order after filing affidavits from two audio experts who maintained the tape had been doctored and a third who concluded the tape's authenticity couldn't be determined without laboratory analysis.

On the tape, Harper can be heard saying he was aware that two Conservative party representatives had discussed "financial considerations" with Cadman as they attempted to persuade him to vote against the minority Liberal government in a crucial confidence vote in 2005.

Zytaruk interviewed Harper while working on a biography of Cadman, which claims that the Tories offered the terminally ill MP a $1-million life insurance policy if he voted against the Liberals.

Liberals pounced on the biography, published last winter, to allege that Harper was aware of attempts to bribe Cadman. That prompted Harper to launch his defamation suit against the party.

Dearden also obtained a court summons compelling Zytaruk to testify at the first hearing in the lawsuit, scheduled for September in Ottawa.

Liberal party lawyer Chris Paliare said Friday that a conference call was held between himself, Dearden and Justice Hackland to hammer out the final details for examining the tape.

He said he could not disclose details of the session, or even identify the expert the Liberal party will retain for its examination of the tape.

Other details hammered out likely include which party's expert will get to examine the tape first.

The court order reveals the legal intricacies involved in what could be an unprecedented examination for a libel suit in Canada.

It spells out that a senior British Columbia lawyer, Murray Clemens, will be appointed, under the authority of the B.C. Supreme Court, to take responsibility for the tape once Zytaruk turns it over.

Zytaruk will have to give Clemens the original tape and the recorder he used to interview Harper. And he will be examined under oath by Clemens about the process he used to record the interview in September 2005, as well as "the chain of possession" of the tape from the time it was recorded to the time he surrenders it.

The court order stipulates that Zytaruk will be paid $250 to appear before Clemens, who must give the author three days' notice to produce the tape. Clemens will also be responsible for taking the tape to the audio experts retained by the Liberal and Conservative parties.

Zytaruk and his lawyer, Barry Gibson of Vancouver, did not respond to telephone calls Friday. Dearden has previously said he has been instructed to refer media questions to the prime minister's office.

Neither side has disclosed the cost of legal work and expert testimony so far in the lawsuit.

Another major court case launched by the Conservative party over the past year -- against Elections Canada in a dispute over Tory advertising expenses in the 2006 election campaign -- has so far cost taxpayers $142,172 for the agency's legal bills.

The cost of a separate investigation by the federal elections commissioner into the disputed advertising expenses has so far cost $504,622, bringing the total cost to taxpayers as of June 30 to $646,794, according to Elections Canada.