TORONTO -- Canada has decided not to participate in a U.S. missile defense system, the government announced yesterday, dealing a symbolic setback to the experimental antimissile project and a blunt rebuff to President Bush, who had personally lobbied for Canada to join.

The decision by Prime Minister Paul Martin, who had earlier signaled he favored signing on to the system, was an acknowledgment of the deep unpopularity among Canadians of Bush and his administration's project to try to shoot down missiles headed toward the United States.

"We will continue to work in partnership with our southern neighbors on the common defense of North America," Martin said. "However, ballistic missile defense is not where we will concentrate our efforts."

His decision has more political than practical impact, since Canada agreed in August to allow its operators at the North American Aerospace Command center in Colorado to share information on incoming missiles, a key concession that had been sought by the United States.

Canada's new ambassador to Washington, Frank McKenna, embarrassed the government Tuesday by acknowledging that the earlier decision meant "we're part of it now."

During a raucous Parliamentary session in Ottawa yesterday, legislators complained that the government was "halfway in and halfway out" of the U.S. program. "What is it that the prime minister has said no to?" demanded opposition leader Stephen Harper. "The government has managed to announce it is in missile defense and not in missile defense in the very same week."

Martin's announcement means Canada will not take part in building the system, paying for it or operating it, even though the plan could call for missiles to be shot down over Canada. Although the United States can continue working on the system alone, Canada's rejection is seen as fuel to U.S. opponents of the project, who voice similar concerns about its cost, practicality and scope.

Martin's decision risks putting a new chill in relations with the Bush administration. Martin had worked hard to improve the relationship after a frosty period that followed Canada's refusal to join the U.S. invasion of Iraq in 2003. As a sign of bilateral progress, Martin hosted Bush on a visit to Ottawa late last year.

But Bush surprised his hosts at the time by pressing hard for Canada's participation in the missile defense plan. Martin was reportedly unable to convince Bush of the political difficulties he would face by joining the U.S. plans.

Canada's public has long opposed any move toward the weaponization of space, and critics have contended that the antimissile program would lead to putting weapons aboard satellites to shoot down other missiles.