A drone was flown so close to a passenger jet that the crew believed it "must have collided with the tail".
The incident occurred as the Airbus A321 flew over Kew, west London on approach to Heathrow Airport.
A three or four-engine white drone passed just five feet over the first officer's window, according to a report by the UK Airprox Board (UKAB).
It stated: "The crew considered that the drone passed close enough that it must have collided with the tail."
Engineers inspected the aircraft after landing but there was "no tangible evidence of collision".
The incident happened at an altitude of 1,700 feet on October 25 last year.
The UKAB put it in the highest category of risk.
"Providence had played a major part in the incident and/or a definite risk of collision had existed," the report concluded.
The identity of the aircraft involved was not disclosed by the UKAB.
In spring the Government is due to publish a draft Drone Bill which will require drone users to register and sit safety awareness tests.
Banning drones from flying near airports or above 400 feet could also form part of the new regulations.