Prime Minister Stephen Harper was cross-examined last week in his $3.5-million libel case against the Liberal party, it was disclosed in an Ottawa court Friday.

Harper agreed to the sworn testimony in his unprecedented suit against the Liberals even though he claimed parliamentary privilege last year to avoid testifying in a libel suit a former Conservative filed against him after the 2006 federal election.

A lawyer for the Liberals confirmed in response to a question from Justice Charles Hackland that Harper had been cross-examined over an affidavit he filed in the case, but she would not reveal any details later other than to say the examination took place last Friday.

Harper's lawyer, Richard Dearden, also declined to comment on the prime minister's testimony.

But Dearden revealed that the next hearing in the case — a potentially explosive court drama that could take place in the midst of an expected federal election campaign — may now have to be postponed.

Former Conservative candidate Alan Riddell sued Harper over statements the prime minister made about an agreement struck between Riddell and the Tories that had him step aside as a candidate for the 2006 election.

As Riddell pressed the suit against Harper, the prime minister argued he could not be compelled to testify when Parliament was in session, citing an 18th-century British law that protected MPs from lawsuits by creditors.

A court master accepted the argument, but Harper later agreed to an out-of-court settlement with Riddell.

Meanwhile, Dearden argued Friday that an expert opinion comparing Harper's suit to one of the biggest scandals in Canadian history should be tossed out of court.

The Liberal party filed the opinion by a political scientist as part of its defence in the defamation suit Harper launched against the party over the Chuck Cadman affair.

The prime minister's reputation has been smeared by the comparison between a 19th century political scandal and allegations Harper was aware Conservatives may have tried to bribe the late MP Cadman, Dearden argued.

The opinion by University of Toronto professor Peter Russell compares the Cadman allegations to the railway scandal that brought down Canada's first prime minister in 1873.

Dearden told Justice Charles Hackland the comparison — which Russell used as one example in arguing Harper is the first Canadian prime minister to sue political opponents — is "absurd" and unfairly taints Harper's reputation.

In the lead-up to another court battle scheduled for Sept. 22 over a journalist's taped interview of Harper that is at the centre of the lawsuit, Dearden argued there is no comparison between the Pacific Railway scandal and allegations that Conservatives offered Cadman a bribe to help defeat the Liberal minority government in 2005.

Dearden stopped short of accusing Russell of attempting to smear Harper by associating him with the Pacific Railway scandal and other controversies, but said: "There is an association, there is a link there."

Harper launched the defamation suit against the Liberals in the wake of a biography of Cadman that quoted his widow saying Conservatives fruitlessly offered Cadman a $1-million life insurance policy to vote against the Liberals in the Commons on a confidence measure.

The prime minister based his lawsuit on Liberal party web postings that suggested Harper was aware Tories had offered Cadman financial considerations, and an audio recording the party posted of an interview Harper had with Cadman's biographer, B.C. author Tom Zytaruk.

Dearden told Justice Hackland that Russell's affidavit contains "scandalous" suggestions that taint Harper's reputation and has no place in arguments over whether the injunction should be issued.

Russell cited the railway scandal as he tried to argue that, unlike any previous prime minister, Harper filed his libel action in an attempt to silence the opposition. He cited another famous scandal that erupted in 1931 over allegations the Liberal party had received large sums of money from a company proposing a massive diversion of the St. Lawrence Seaway for a hydroelectric project.

It emerged that the company received a bill from a Liberal senator for payment of the cost of a holiday former prime minister Mackenzie King had in Bermuda.

Dearden said the examples of scandals where misconduct was discovered has no bearing on Harper's case.

"It's scandalous to do that," Dearden argued, calling it an "absurd and ludicrous" proposition to compare the earlier scandals with the Cadman affair.

He also asked Justice Hackland to strike out portions of Russell's essay that accused Harper of launching the libel suit to force an end to political debate over the bribery allegations.

Though Harper earlier had insisted on a speedy hearing of his request for a court order to stop the Liberals from distributing or quoting the Zytaruk tape, Dearden complained Friday about delays receiving records from the Liberal legal team and revealed a sound expert Harper has retained to examine the tape will be on a European holiday for two weeks prior to the Sept. 22 hearing.

Lawyers for the Liberals filed a 14-page essay from Russell as expert evidence. The essay says it would be a blow to freedom of expression for politicians and free discourse in politics if Harper succeeds.

Justice Hackland questioned Dearden several times about Harper's objections to the affidavit and suggested it might be best to let the judge who will hear the injunction application next month decide.

A Liberal lawyer at the hearing, Odette Soriano, said Russell's viewpoint is relevant once the case reaches a stage where Harper's libel allegations are argued.