Canada is set to announce that it will not take part in the US's ballistic missile defence system beyond existing commitments under the North American Aerospace Defence Command (Norad).

The announcement, expected within the next few days, will be widely interpreted as a rebuff to President George W. Bush, who has urged Canada to sign up for the system, on which Washington is due to spend more than $50bn (€38bn, £26bn) in the next five years.