A B.C. judge has ordered a Vancouver reporter to reveal the name of an anonymous source in a libel lawsuit that involves a former Liberal-turned-Green MP.

The Vancouver Province newspaper says it will appeal the decision, and plans to seek a stay of the ruling in the meantime.

The defamation case stems from a complicated family dispute between Kelly Wilson, the wife of former Vancouver-area MP Blair Wilson, and her stepfather, Bill Lougheed.

The Vancouver Province newspaper published an article in which Lougheed alleged Wilson was "not fit for public office" and also included allegations from an anonymous letter-writer that Wilson had spent too much money on his campaign, B.C. Supreme Court judge Paul Williamson notes in his ruling.

The letter was also sent anonymously to Elections Canada and a political blogger, but the ruling notes only the Vancouver Province reporter Elaine O'Connor appears to know the name of the source. O'Connor says she promised to keep the letter-writer's identity secret.

The letter triggered an investigation by Elections Canada, which eventually concluded most of the allegations were unsubstantiated. Wilson admitted only to failing to disclose both a donation of umbrellas and $9,000 in campaign expenses.

The news coverage prompted the Liberals to boot Wilson from caucus, and he later joined the Green party before he was defeated in the 2008 federal election.

Wilson sued for defamation and demanded the identity of the newspaper's anonymous source. The defamation case is currently in the pre-trial discovery phase.

The judge pointed to a landmark Supreme Court of Canada decision from last year, in which the top court declined to create a blanket protection for journalists who rely on anonymous sources. Instead, the court set out criteria judges must consider when weighing whether a source should be protected.

One of those criteria is whether the public interest is better served by revealing a source or keeping them confidential.

In this case, Williamson said he can't make that determination without knowing whether the source was motivated by a sense of civic duty to reveal possible wrongdoing, or whether he or she was attempting to defame Wilson for personal gain or malice.

The only way to know for sure, writes Williamson, is to know who they are.

"If the motive of the source was to disclose information concerning the integrity of our system of government, then the protection of the identity of the source triumphs," the judge writes in a decision posted to the court's website.

"But if the source released the letter to the journalist based upon intentions that are personal, malicious and vindictive, his or her identity should be disclosed."

Williamson ordered O'Connor to disclose the identity of the source and to outline what communications she had with the source.

The Vancouver Province's editor-in-chief, Wayne Moriarty, said the newspaper will appeal.

"This ruling should concern journalists and their sources," Moriarty said in an interview.

"The Supreme Court (of Canada) has been consistent in not undermining the principles of journalistic confidentiality, and knowing that, I have every confidence that we'll win on appeal."

Wilson's lawyer, Jay Straith, acknowledged there are serious free-speech issues at play, but he said that must be weighed against the damage done to his client's reputation.

"There's a difference between somebody tipping off a reporter that there's been some improper behaviour by someone in public life, as opposed to ... if it turns out to be a political partisan," Straith said in an interview.

Straith noted the court order is only for the reporter to turn over the source to the judge for discovery. Whether it will eventually be made public is an issue that will be sorted out later, said Straith.