Canada has decided not to take part in the United States' planned North American missile defense system.

Canada's decision was passed along at the NATO summit in Brussels this week, ahead of an expected announcement from Prime Minister Paul Martin Thursday.

President Bush had urged Canada to sign on to the system, still in the testing stage, which would aim to shoot down incoming missiles while still in the air.

The system has been the subject of contentious debate in Canada's House of Commons, where Mr. Martin's Liberal Party recently lost its majority.

The U.S. ambassador to Canada says Washington is perplexed by Ottawa's decision.

Last August, the two countries agreed to share data on missiles flying over North America, information the U.S. military could use to target incoming missiles.