A labour dispute triggered massive delays at Canada's busiest travel hub on Thursday as security personnel at Pearson International Airport launched a work-to-rule campaign and slowed security lines to a crawl.

An official with the Greater Toronto Airport Authority confirmed the work slowdown via Twitter on Thursday as security line ups continued to spread through Pearson's Terminal 1.

Travellers at the scene say they have been forced to wait two to three hours to get through customs. Numerous flight delays have been reported as a result.

Security services provider Garda says their workers are intentionally slowing down the screening process to protest changes to their work schedules.

The workers returned to their normal work levels at about 3 p.m., when the Canadian Industrial Relations Board ordered the work-to-rule campaign stop.