A Federal Court decision released on Tuesday gives Canada's spy agency the authority to eavesdrop on Canadians overseas.

Justice Richard Mosley revealed that the Canadian Security Intelligence Service (CSIS) obtained a warrant to monitor two suspects considered threats to Canada late last year.

When in January of this year, it was discovered the pair were leaving the country, CSIS won court approval to employ the secretive Communications Security Establishment, a wing of National Defence, to ensure the interceptions could continue.

An earlier court ruling had made it clear that CSIS could not legally carry out eavesdropping on foreign soil without approval from the country in which the spying takes place.

But with the help of CSE's technical means, the interceptions would be "controlled from within Canada," Mosley said, making the operation legal.

"Information which may be crucial to prevent or disrupt the threats may be unavailable to the security agencies of this country if they lack the means to follow those lines of communication," Mosley wrote in his decision.

The Ottawa-based CSE collects and processes telephone, fax and computer communications of foreign states, corporations and individuals. The federal government uses intelligence gleaned from the interceptions to support troops abroad, catch terrorists and further Canada's economic goals.

CSE is generally prohibited from spying on Canadians, but it can assist CSIS and police agencies acting under judicial warrants.

Mosley argued in his decision that although CSIS planned to enlist CSE in intercepting the telephone calls of Canadians travelling outside the country, the actual eavesdropping would take place "at the locations within Canada where the calls will be acquired, listened to and recorded" and thus was legal.