TORONTO - Undercover police officers who pose as journalists for investigative purposes are violating the Constitution by having a chilling effect on freedom of the press, an Ontario court heard Monday.

In their application to Superior Court, three media organizations argue the deceptive practice could put working journalists at risk, especially in high-stress environments, by raising suspicion about who they are.

The practice can also make it harder to win the trust of important sources and therefore get key information that is in the public interest, they say.

"This is very destructive of everything our clients do," media lawyer Philip Tunley told the court. "This chill is a real and substantial one."

The media organizations — the CBC, Canadian Journalists for Free Expression, and RTDNA Canada — want the court to declare impersonation of journalists by Ontario Provincial Police a charter violation that can't be justified.

Such a declaration, they say, would "begin to thaw the chilling effect" the practice has had on the public, sources of important news and information, and on journalists.

"A strong commitment to free expression requires nothing less," they argue in their court filings.

The organizations cite three examples — some going back more than a decade — as proof provincial police engage in posing as members of the media to gather information as part of a criminal investigation and the targets are usually aboriginals or other vulnerable groups.

In one notorious case, two officers filmed protesters at Ipperwash provincial park in 1995 and, when asked who they worked for, named the fictitious United Press Associates. Police confirmed the deception years later at a public inquiry into the fatal police shooting of an aboriginal protester.

In another case, an officer keeping an eye on protesters during an aboriginal Day of Action on Tyendinaga Mohawk territory in 2007 admitted he had pretended to be part of the media, court heard.