Prime Minister Stephen Harper says his government will introduce a bill next week to establish fixed federal election dates every four years except in cases where the government is defeated in a House of Commons vote or is otherwise "prevented from governing."

The next vote would be in the fall of 2009, he said in a speech in Victoria on Friday.

The proposed bill would abandon an age-old British practice followed in Canada since Confederation. As things stand, a government can carry on for as many as five years without an election — the period set out in the 1982 Constitution — but the prime minister can call an election at any time within that span.

Harper said British Columbia, Ontario and Newfoundland and Labrador have already moved toward fixed dates. U.S. presidential elections are held every four years.

"Fixed election dates prevent governments from calling snap elections for short-term political advantage," Harper said. "They level the playing field for all parties and the rules are clear for everybody."

Because the government could be defeated in the Commons before the end of a four-year term, "the will of a majority in Parliament will always prevail," he said.

"But fixed election dates stop leaders from trying to manipulate the calendar simply for partisan political advantage."

He said he was willing to give up a prerogative traditionally enjoyed by sitting prime ministers.

"I read the polls saying if I called an election now we would win a majority. The same polls also say no one wants an election now, and no one does want an unnecessary election. So unless we're defeated or prevented from governing we want to keep moving forward to make this minority parliament work over the next 3½ years."

He did not describe the circumstances in which he would consider the government to be prevented from governing. Among other things, he may have been thinking of opposition parties obstructing House business by procedural means.

Speaking to reporters after the speech, Harper stressed that he was not seeking a pure fixed-date system. "If the government is defeated, loses confidence, it's obliged by the constitution to hold an election."

He added: "A federal campaign is very difficult thing to organize, so this is a useful advantage that we're willing to give up."

Because the change would be done through a law passed by Parliament, future governments would be free to change it back.