OTTAWA - Watch for political ads coming soon to the side of a bus near you.

British Columbia transit officials were on the wrong side of the Charter when they refused to carry messages on the sides of their buses aimed at provincial voters, the country's top court said Friday.

The Supreme Court of Canada struck down transit policies banning all political ads, saying they violate rights to free speech.

"Like a city street, a city bus is a public place where individuals can openly interact with each other and their surroundings," wrote Justice Marie Deschamps in the 8-0 ruling.

All nine judges heard the case in March 2008 but Justice Michel Bastarache has since retired.

"I do not see any aspect of the location that suggests that expression within it would undermine the values underlying free expression," Deschamps wrote. "On the contrary, the space allows for expression by a broad range of speakers to a large public audience.

"I therefore conclude that the side of a bus is a location where expressive activity is protected by ... the Charter."

The judgment was being watched by cities across Canada that have so far rejected atheist bus banners declaring: "There's probably no God. Now stop worrying and enjoy your life."

The ruling protecting political ads will be viewed as a boon to those hoping to buy bus space for their atheist message.

Friday's ruling is a victory for the Canadian Federation of Students and the British Columbia Teachers' Federation.

Both groups tried to place ads on the outside of B.C. buses leading up to the provincial election in 2005.

They raised several issues including voter turnout, tuition fees, the environment and school closures.

The student federation had hoped to run an image of a crowd at a concert with the text: "Register now. Learn the issues. Vote May 17, 2005."

The teachers union wanted to post a banner on the side of buses saying: "2,500 fewer teachers. 113 schools closed. Our students. Your kids. Worth speaking out for."

British Columbia Transit and TransLink refused to run them, saying policy bars political ads along with any message "likely to cause offence ... or create controversy."

The high court pointedly noted that bus officials had no problem running commercial ads. It also said their policy attempting to vanquish all controversy "is unnecessarily broad."

"Citizens, including bus riders, are expected to put up with some controversy in a free and democratic society."

Still, there are times when messages in publicly governed spaces can be justifiably restricted, Deschamps wrote. The fact that buses are used by an essentially captive audience, including children, must be considered.

"Thus, limits on advertising are contextual."

The Canadian Code of Advertising Standards "could be used as a guide to establish reasonable limits ... on discriminatory content or on ads which incite or condone violence or other unlawful behaviour.

"But the determination of what is justified will depend on the facts in the particular case."

At trial, the judge found that transit bodies, as publicly controlled government entities, must uphold the Charter of Rights and Freedoms. But he cited the fact that transit company policy had never allowed political ads, and did not define that as a breach of free expression.

The B.C. Court of Appeal ruled 2-1 against him, and struck down any blanket ban on political messaging as unconstitutional.

Dissenting B.C. Justice Mary Southin said political bus ads don't qualify as the type of expression protected by the Charter. Forcing transit authorities to display such messages would be like obliging newspapers to publish opinion pieces, she wrote.

Several cities including Vancouver, Victoria, Halifax and London, Ont., initially rejected the atheist "There's probably no God" ads.

That message has been placed on public transit systems in several countries including the U.S., Italy, Spain, the Netherlands and other parts of Europe since last year.

The atheist campaign began in Britain when a woman was annoyed that an ad posted in a London bus linked her to a website warning that non-Christians will "spend all eternity in torment in hell." She decided to raise money for a rebuttal.

The atheist bus message movement was born -- and it soon picked up speed.

Toronto, Calgary, and Montreal accepted and ran the "There's probably no God" ads, but they hit a roadblock in Ottawa. The capital's transit service OC Transpo rejected them at first.

The company wound up displaying the message on the side of its buses last spring after Ottawa city council cast a split vote to overturn the refusal amid concerns about a costly legal fight.