An illegally operated drone came within metres of colliding with a passenger jet landing at Heathrow .
The close shave, which investigators placed in the most serious risk category, was the latest in a spate of near misses involving the popular remote controlled flying devices.
The Airbus A319 was flying at 500ft on its final approach to the airport on September 30 when the pilot spotted the drone, which he estimated passed just six metres above and 23 metres to the left of the plane.
He told the UK Airprox Board (UKAB), which investigates near misses, that it was not possible to take evasive action.
Officials said the drone was operated against Civil Aviation Authority (CAA) regulations, as permission had not been granted for it to fly above 400ft within the Heathrow CTR control zone.
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They concluded that the separation between the drone and the jet had been reduced to about a wingspan - described as "the bare minimum" - and that "chance had played a major part" in the avoidance of a collision.
The drone did not show up on radars and although the incident was reported to police the operator could not be traced.
Steve Landells, flight safety specialist at the British Airline Pilots Association, has previously called for designers to look at ways to make drones visible to air traffic controllers.
UKAB's latest monthly meeting on near-misses analysed a total of six possible incidents between aircraft and drones.
They included a drone reportedly passing within 15 metres of a short-haul jet close to Manchester Airport in October.
UKAB previously published details of seven drone near-misses following its December meeting, including a drone coming within metres of colliding with a jet above the Houses of Parliament on September 13.
