Canada is set to announce that it will not take part in the US's ballistic missile defence system beyond existing commitments under the North American Aerospace Defence Command (Norad).

The announcement, expected within the next few days, will be widely interpreted as a rebuff to President George W. Bush, who has urged Canada to sign up for the system, on which Washington is due to spend more than $50bn (€38bn, £26bn) in the next five years.

Paul Martin, the Canadian prime minister, said last year that Canada should be involved in the initiative because it was designed to protect the entire North American continent.

However, Mr Martin's change of heart is mitigated by a deal last year under which Canada agreed to continue, through Norad, to track incoming enemy missiles and aircraft.

US and Canadian forces are closely integrated in Norad. But the details of what the US expected of Canada are unclear.

David DeWitt, director of the Centre for International and Security Studies at York University in Toronto, said: “We've already signed on in the most important areas.”

Ottawa's new ambassador to Washington, Frank McKenna, said this week that last year's agreement meant that Canada was already in effect part of the missile defence system.

Mr Martin has been forced to change tack by determined opposition within his Liberal party, which won only a minority of seats in the House of Commons in June. A motion opposing missile defence is due to be submitted to a party policy convention this month.

Doubts on Canada's participation are especially strong in Quebec, where the Liberals hope to improve their showing in the next election.

The US was informed of Canada's decision at the Nato summit in Brussels on Tuesday, attended by Mr Martin and President Bush, according to Canadian media.

The missile defence debate has tapped into a growing nationalistic streak among many Canadians, fanned by wide dislike of President Bush's policies. Mr Martin's predecessor, Jean Chrétien, won strong support two years ago for his decision not to send Canadian troops to Iraq.

Canadian support for missile defence has been further eroded by doubts about the efficacy of the US system. An interceptor rocket failed to launch from a base in the Pacific Ocean this month, the third such setback.

The missile defence programme, which was meant to be operational by this autumn, is running well behind schedule.