Canada could be headed for a constitutional showdown after opposition parties passed a bill requiring the government to meet its commitments to Kyoto, and Ottawa has hinted it may ignore the legislation.

Introduced by a Liberal backbencher and backed by the NDP and the Bloc Québécois, the private member's bill easily passed a House of Commons vote Wednesday evening, 161-113. It is expected to be passed by the Liberal-dominated Senate.

Bill C-288 gives the government 60 days following royal assent to table a detailed plan outlining how Canada will reduce its greenhouse gas emissions under the commitments of the Kyoto Protocol. Canada ratified the international agreement in 2002 under the former Liberal government.

However, Prime Minister Stephen Harper, who didn't attend the vote, has said meeting the 2012 targets would cripple the economy, delivering a multibillion-dollar hit to businesses and consumers over several years.

The government has hinted strongly that it could simply ignore the law, and would be prepared to face any resulting lawsuits or even a non-confidence motion that could trigger an election.

Environment Minister John Baird has called the proposed legislation a "toothless tiger," saying it has no penalties if the conditions of the bill aren't met.

"This is a don't-worry-be-happy bill that says, 'Table a plan in 60 days,'" said Baird. "If it was so easy, I suspect the Liberals would have done it when they had the opportunity."

Under the former Liberal government, greenhouse gas emissions rose more than 25 per cent.

Government must bow to will of Parliament: Dion

But Liberal MP Pablo Rodriguez, who introduced the bill, said the government can't pick and choose which laws it follows, saying Harper "can't act like an emperor."

Liberal Leader Stéphane Dion said the government has no choice but to respect the will of Parliament.

"In our system, Parliament is the legislative body and the government is the executive body. The executive cannot ignore what we have as legislation in our country," said Dion. "This is the law of the land."

Speaking before the vote, Rodriguez said his party would be willing to "go to the courts" if the government doesn't implement the requirements of the bill, while Liberal MP Michael Ignatieff said he wouldn't rule out tabling a non-confidence motion on the issue.

NDP environment critic Nathan Cullen said he doesn't know whether the NDP would join in any future legal challenge if the government ignores the bill.

Cullen said the Liberal leader in the Senate told him there are no plans to fast-track debate on the bill and that it could take six months to a year before it is passed.

Several constitutional experts have agreed the Conservatives must respect the law.

In interviews with Canadian Press, university law professors Ned Franks, Patrick Monahan and Stewart Elgie all agreed that the government has no choice but to follow the law.

NDP Leader Jack Layton said it would be hypocritical for Harper to ignore the bill if it becomes law.

"I think Mr. Harper is turning his back on what he called for when he was in opposition," Layton said. "He said [then] that the government, particularly in a minority government, had to respect … the democratic will of the House of Commons."

During the last federal election campaign, Harper urged the Senate not to interfere with the "democratic will of the elected House." 

With files from the Canadian Press