The aircraft had been operating a service from Barcelona on February 19 and was flying at 5,500 ft when the co-pilot saw the drone.
He immediately disengaged the auto pilot and carried out an avoidance manoeuvre while informing the captain of the drone's presence, BEA said on its website.
The drone passed just five metres below the aircraft's leftwing, the agency said, classifying the incident as "serious".
An investigation is still ongoing.
- Reuters