The 28-year-old co-pilot started speaking gibberish as she tried to read the checklist for landing. She then twisted violently in her seat, inadvertently moving the rudder on the Boeing 737, the report said.
The captain, Renny Manclark, 46, adjusted the rudder, called for assistance and put out a mayday call requesting priority landing, the Air Accidents Investigation Branch report said.
Cabin crew had great difficulty removing the unconscious co-pilot from her seat without interfering with the captain's control of the aircraft. They managed to get her into the forward galley area and the captain landed safely.
The co-pilot was transferred to hospital where she recovered after several hours. "Her medical condition is still being assessed," the report said.
The incident was at 6pm on 5 January 1999, on a flight from Helsinki.
BA said yesterday that the co-pilot was doing ground work, pending the result of medical tests. "The captain and his cabin crew colleagues handled this incident impeccably and at no time were passengers in any danger."
Captain Manclark, a former RAF pilot, said: "We always hope something like this never happens to us, but we are trained to deal with such situations."