The move represents an about-face for Prime Minister Paul Martin, who in the run-up to a federal election in June said he thought Canada should be part of a system designed to protect the North American continent.
But Martin lost his parliamentary majority in that election and is now struggling to keep his minority government afloat with the support of a smaller left-leaning party that is strongly opposed to missile defence.
"The prime minister plans to announce this week that Canada will not be a part of the missile defence system," a senior official said on Tuesday.
Canadian media quoted their own federal government sources as saying Canada would not take part.
The US State Department had no response to the news.
Making matters more uncomfortable for Martin is the fact that his ruling Liberal Party is split over the idea of signing on to the system, and the government already faces a battle to ensure its first budget - to be presented on Wednesday - is not rejected by parliament.
Bush visited Ottawa last December and publicly urged Martin on three occasions to sign on to a network designed to fend off attacks from "rogue states" such as North Korea.
The New Democrats say missile
defence could spur an arms' race
The Canadian decision marks the second time in less than two years that Ottawa has gone against Bush's wishes.
In March 2003, former prime minister Jean Chretien enraged Washington by refusing to take part in the invasion of Iraq.
Martin took over from Chretien in late 2003 and promised to repair strained ties with the US.
Ottawa has been holding talks with Washington for the past 18 months on whether to formally join the proposed network and officials had let it be known there were few obstacles to Canadian participation.
But the prospects for a final agreement dimmed after the June election.
Former premier Chretien refused
to take part in Iraq's invasion
Martin's survival now depends in large part on the minority New Democrats, who oppose missile defence on the grounds that it could lead to a new arms race.
The system is also unpopular in French-speaking Quebec, a powerful province where the Liberals will need to boost flagging support if they are to win the next election.
Earlier in the day, Ottawa's new ambassador to the United States caused uproar by saying that a pact the two neighbours signed last year on boosting North American defence cooperation and intelligence sharing meant Canada was effectively already part of the missile defence system.
Opposition politicians angrily accused Martin of deception, saying he had promised to allow the House of Commons to debate the matter first.
"How could this prime minister secretly make this decision, clearly breaking every commitment he's made to this House and to Canadians?" demanded Stephen Harper, leader of the official opposition Conservative Party.