The environment -- a topic the opposition sees as the Conservatives' weak spot -- is set to be a key issue in the next federal election, which some political observers and insiders say could be called in the next few months.

The House of Commons adopted the bill on Wednesday in the latest clash over green issues between opposition legislators and the minority government, which says Canada cannot meet its Kyoto targets.

The Kyoto bill must now go the Senate, Parliament's upper house, for approval before formally coming into force. The Senate is dominated by members of the opposition Liberal party and officials said the law could be adopted in April or May.

The Conservatives describe the bill as "a toothless tiger" because it does not provide any money to implement the cuts called for by Kyoto.

"If and when that becomes law, the government would respect it. I'll just point out ... that the bill has no plan of action in it, the bill gives the government no authority to spend any money to have a plan of action," Harper told Parliament.

The opposition Liberals said Ottawa could be sued if it carried out earlier threats to ignore the law and they attacked what they said was Harper's contempt for Parliament.

"Is the prime minister of Canada now going to decide which laws he's going to respect and which laws he's going to break?" asked Pablo Rodriguez, the Liberal legislator who drew up the draft legislation.

"Is he going to throw the foundations of our democracy to the grounds and close Parliament? Is he going to say 'Democracy is finished, long live the dictatorship'?" he asked Parliament after Harper had spoken.

The law would give the Conservatives 60 days to come up with a plan to meet the cuts laid out by Kyoto -- reductions that Harper says would paralyze the Canadian economy. Canada is the largest exporter of energy to the United States.

Environment Minister John Baird said he was more interested in pushing ahead with the government's own clean air plan that was unveiled last September.

The plan -- which did not call for binding emissions cuts until 2020 at the earliest -- was savaged by critics and is now being rewritten by a special parliamentary committee.

Kyoto obliges Canada to cut emissions of greenhouse gases blamed for global warming by 6 percent from 1990 levels by 2012. Emissions are currently 27 percent above 1990 levels.

The environment is also a challenging issue for the Liberals because they were in power from 1993 to early 2006 -- a period when emissions surged.

The party's record on green issues is under attack both from the Conservatives and the left-leaning New Democrats, who are eager to pick off soft Liberal votes in the next election.