Most of Canada's major broadcasters are cracking down on political parties using their material in advertising.

In a letter sent to federal and provincial political parties today, representatives from CBC, Radio Canada, CTV, Rogers, and Shaw — which owns Global Television — said they would no longer air political advertisements that include material taken from their airwaves without their express authorization.

The Conservatives used footage of Liberal Leader Justin Trudeau taking his shirt off at a charity fundraiser in TV ads last year. The Liberals complained about use of copyrighted materials in the ads. (Conservative Party of Canada advertisement) "As news organizations, the use of our content in political advertisements without our express consent may compromise our journalistic independence and call into question our journalistic ethics, standards and objectivity," the broadcasters wrote.

Broadcasters have for years complained to political parties about this practice, but to no avail. Instead of taking the political parties to court for copyright infringement, the TV companies have mutually agreed not to air material that offends their standards.

Quebecor, the company that owns Sun News Network and TVA, is the only major broadcaster not to sign the letter. Quebecor did not respond to CBC News' questions about whether it was invited to sign the letter.

CBC management declined to comment on the letter, while calls to other broadcasters were not returned.

Last year, Liberal MP Stéphane Dion sent a letter to Elections Canada complaining about Conservative ads that used material from the Huffington Post and CTV to attack Liberal Leader Justin Trudeau.

In the letter, Dion said the ads contained "unauthorized use of copyrighted materials" in violation of Canadian copyright laws.

Dion also noted the ads were being aired nationally, including in Labrador where a federal byelection was underway, and argued that using material that would normally require a licence fee should be considered a "non-monetary contribution" by a corporation to the Conservative Party and therefore prohibited by Canada's elections law.