Investigators are inspecting an Air Canada Rouge Boeing 767-300ER after it experienced the uncommanded transfer of fuel between tanks.
The aircraft (C-GDUZ) had been operating from Athens to Montreal on 16 July.
Transportation Safety Board of Canada says the twinjet, powered by General Electric CF6 engines, had been climbing to cruise altitude when the crew identified a "slight" fuel imbalance.
The crew carried out actions from the quick-reference handbook to try to rectify the situation.
But the board says the pilots subsequently determined that an uncommanded transfer of fuel – at the rate of 2.6t/h – was taking place between the left-hand main tank and the centre tank.
The aircraft climbed to around 32,000ft but, as a precaution, diverted to London Heathrow where it landed at about 18:15.
None of the 257 occupants was injured. The board says the twinjet has been undergoing inspection by the carrier's maintenance personnel.