Prime Minister Paul Martin was expected to announce later in the week that Canada will not sign on to the U.S. ballistic missile plan, after his newly appointed ambassador to Washington triggered a row by suggesting Ottawa (search) had already joined the contentious program.

Most Canadians are opposed to the missile defense shield, which is in the midst of testing interceptors capable of taking out incoming missiles. Some believe the umbrella, when fully implemented, could lead to the weaponization of space and an international arms race.

The Foreign Ministry insisted Ambassador Frank McKenna's (search) comments on Tuesday referred to a previous military agreement, not the proposed missile shield. That was not good enough for opposition politicians, who said Martin had betrayed Canadians by "secretly" signing on to the shield program.

A spokesman for the U.S. Northern Command (search), Michael Kucharek, said there has been no change in Canada's involvement in U.S. ballistic missile defense programs.

Radio-Canada television and the Canadian Press agency (search) later reported that Martin told NATO allies in Brussels on Tuesday that Ottawa would not join the program.

Martin had promised a new era of Canada-U.S. relations after bitter divisions over the war in Iraq, and Americans have warned that relations would deteriorate further if Canada refused to join the missile plan.

But polls indicate that nearly two-thirds of Canadians are opposed to the program, and the opposition both inside and outside Martin's Liberal Party appear to have made it impossible for Martin to move forward, government officials said.

The United States was informed of Canada's plans at the Brussels summit, attended by Martin and President Bush, and the news was also conveyed Tuesday through diplomats in Ottawa and Washington, Canadian Press reported.

Canada receives missile-warning data through the North American Aerospace Command, a joint U.S.-Canadian military operation that dates back to the Cold War to monitor missiles, aircraft and space objects and warns of threats to the continent. But Canada is not part of the existing missile defense shield, and the Foreign Ministry says McKenna's comments referred solely to the NORAD agreement.

A seasoned politician and former premier of New Brunswick, McKenna was chosen for the Washington post to enhance relations between the prickly North American neighbors. But even before his scheduled arrival in Washington next month, McKenna told the foreign affairs committee in Ottawa that he didn't understand why President Bush had implored Martin to sign on to the program when he visited Canada in December.

"We're part of it now and the question is, what more do we need?" McKenna later told reporters when asked to clarify.

Martin has said Canada would not support the U.S. missile defense plan if it meant putting weapons in space or loss of control over Canadian air space. He has since remained vague on the issue, saying he didn't know what kind of program Bush has in mind.

Although recent tests have been unsuccessful, U.S. military officials say they could fire interceptors at incoming missiles from bases in Alaska and California. They have stopped short of declaring the system "on-alert," and missiles currently in the silos are prevented from launching.