TORONTO - Ontario's new police commissioner says it's against police policy for officers to pretend to be journalists to gather evidence on suspects, but he can't rule out it happening again.

Chris Lewis said it would only happen in "rare" cases where public safety is an issue.

"To save a life to get close to that person, then we might do what we have to do," Lewis said Tuesday as he officially took over from retired Ontario Provincial Police commissioner Julian Fantino.

Media advocates are launching a court action this fall against the practice, saying it undermines the public's trust in reporters.

At first Lewis said his officers would no longer pose as reporters, citing a policy brought in three years ago that stopped the practice.

But he quickly added that he couldn't rule out "every exigency in the world" and that exceptions could arise.

In 2007, an OPP officer pretended to be a journalist at a Mohawk rally held in conjunction with the Aboriginal Day of Protest.

Recently an officer posed as a journalist to gather evidence from an inmate in a prison.

Three Quebec provincial police officers posed as protesters at the summit of the three North American leaders in Montebello, Que., in August 2007.

The group Canadian Journalists for Free Expression is aware of a handful of cases in Ontario, but called it the "tip of the iceberg."

Police across Canada employ the tactic, Arnold Amber, CJFE's president, said in an interview Tuesday.

"We understand that the Mounties do it, the OPP does it, the Quebec police do it," said Amber.

"Every way you look at this here it is a lousy practice."

The CJFE, the CBC and RTNDA Canada plan to launch the court challenge in the Ontario Superior Court of Justice against Ontario Provincial Police and the Ministry of Community Safety and Correctional Services.

Sources might not talk to reporters if they think they may be speaking to someone who's really a police officer, so important stories might not get told, said CJFE manager Julie Payne.

Pedophile priests at the Mount Cashel orphanage and the Robert Pickton serial killings are two cases in which police investigations were launched only after stories appeared in the news media.

The most recent reports of a provincial police officer posing as a reporter emerged this weekend. The officer gathered evidence against Phillip Vince in the July 1999 cyanide poisoning of fellow inmate Scott Barnett at Millhaven Penitentiary in Kingston, Ont.

"You had a police officer pretending to be a journalist who was going to write about this prison inmate in order to get him to potentially give evidence of a crime he had committed," said Payne.

"And the police officer as that journalist made all sorts of promises that the conversations were to have journalists privilege and wouldn't be shared with police."

Vince, 52, was arrested at the Saskatchewan Penitentiary in March and charged with first-degree murder.

Vince had been serving a life sentence for two first-degree murders in Durham Region in 1985.

The CJFE has asked police to stop the practice before. It sent a letter in July 2008 to Ontario's minister of community safety and corrections asking that the process be stopped.