"We are aware of the occurrence, we are investigating to get all the facts," John Cottreau, a spokesman for the Transportation Safety Board of Canada, told AFP. "The plane was on route to Tokyo... it landed back without incident."
Cottreau refused to confirm a report by public broadcaster CBC that one of the plane's engines had shut down shortly after takeoff.
Air Canada Flight 001 had left Toronto at 2:10 pm (1810 GMT) with 318 passengers on board and 16 crew. It landed back in Toronto at 3:53 pm.
Some reports indicated that witnesses had told police that they had seen debris fly off of the plane.
Air Canada was not immediately available for comment.