A passenger aircraft narrowly avoided a collision with a drone over central London, according to a report.
The Airbus A320, which was carrying around 165 passengers as it approached Heathrow Airport on 18 July, was within 66ft (20m) of the device.
The report, based on the pilot's observations, described the incident as a "very near miss".
The plane was at an altitude of 4,900ft (1,494m) when the drone was seen from the right-side flight deck window.
The crew said the drone - described as black and about 50cm (20in) wide - "probably" passed above the right wing and a horizontal stabiliser, which is found on the tail of the plane.
The UK Airprox Board report said: "Members agreed that this incident appeared to be a very near-miss and that the drone operator should not have been flying in that location at that altitude."
It added that the account of the incident "portrayed a situation where a collision had only been narrowly avoided and chance had played a major part".
The incident was deemed to be in the "most serious" category of risk. The drone operator has not been traced.