The Boeing 747 had been descending through 3.500 ft approaching at London Heathrow runway 27L on June 6th.
The crew clearly saw a "yellow and orange quadcopter" pass down the left side of the aircraft, with a vertical separation of just 20 ft and horizontal separation of 40 ft.
Investigators of the Airprox Board classified the incident as high-risk.
The drone operator has not been identified by the investigators.
Source: FlightGlobal