Suddenly, the pilot was startled by a small black object flying over the aircraft's wing, between 20 and 50 feet away.
He radioed air traffic control, reporting "some kind of balloon or drone, small drone-type object, that flew over us."
Radar showed nothing. Other aircraft in the area were notified, but the object was not seen again. The Airbus landed safely.
The pilot described the object as rectangular and apparently propeller-driven, "like a drone," according to the UK Airprox Board, which investigates near misses in Britain.
While that body was unable to determine definitively whether the object was a drone or a balloon, the UK Civil Aviation Authority
is convinced it was a drone -- an unmanned aircraft -- piloted for sport by remote control from the ground.
Efforts to find the drone operator failed. But investigators classified the incident, which took place March 15, as a "category A," the most serious classification, in which the risk of collision is high, according to the Civil Aviation Authority.
'One of the busiest airspaces in the world'
On Wednesday, the Civil Aviation Authority publicly warned drone operators to follow the rules or face prosecution.
"Drone users must understand that when taking to the skies they are entering one of the busiest areas of airspace in the world -- a complex system that brings together all manner of aircraft including passenger airplanes, military jets, helicopters, gliders, light aircraft and now drones," Tim Johnson, the aviation authority's director of policy, said in a statement.
"When doing so, they must be aware of the rules and regulations for flying drones that are designed to keep all air users safe."
Interest in drones has increased rapidly in the last couple of years, Johnson said, and it is essential that drone operators have regard for other users of the airspace.
How to ensure safety? "Keeping your drone within sight is key," a release from the aviation authority advises.
And it appears that this may have been precisely what the operator of the drone seen by the Airbus A320 -- if indeed it was a drone -- failed to do.
'A marked increase' in near misses with drones
Because of the altitude, the Airprox Board said, the object was unlikely to have been a drone piloted by someone on the ground who was steering it by looking at it.
"The possibility exists that is may have been a drone controlled by 'First Person View," the investigatory body said.
In other words, it may have been a drone mounted with a camera, allowing the pilot on the ground to fly it without being able to see it, relying instead on live video from the device.
And the Airprox Board said that steering by video alone is a growing problem.
There has been "a marked increase" recently in the number of near misses involving drones, the Airprox Board said.
"A common theme through these recent reports is that the encounters have been reported at altitudes above 1,500 feet, which is in almost all cases well beyond a height that the person flying the 'drone' will be able to maintain visual contact with it, and the airspace around it," the investigatory body said.
And that, the board said, is against the rules.