Toronto ? Prime Minister Paul Martin said Thursday that Canada would not join the contentious U.S. missile defense program, a decision that will further strain brittle relations between the neighbors but please Canadians who fear the program could lead to an international arms race.

The Bush administration has tried to emphasize that Martin heads a minority government that could fall over such a contentious debate. But after the announcement, U.S. Ambassador Paul Cellucci said he was perplexed by Canada’s decision, which he said effectively allowed the United States to decide what to do if a missile were headed toward Canada.

“We simply cannot understand why Canada would in effect give up its sovereignty — its seat at the table — to decide what to do about a missile that might be coming towards Canada,” said the outgoing ambassador, who had vigorously urged Canada to sign on the plan.

Martin, ending nearly two years of debate over whether Canada should participate in the development or operation of the multibillion-dollar program, insisted his decision had not relinquished Canada’s sovereignty over its airspace and that Canadian officials would expect to be consulted what to do about any missile passing over their country.

“We are certainly intending to defend our sovereignty and our air space and if anything develops in our air space, we expect, as a sovereign state, to be notified and have influence on any decisions,” he said. “Canada’s a sovereign nation and we would expect and insist on being consulted on any intrusion into our air space.”