UK aviation authorities are investigating claims by an airline pilot that his plane was almost struck by a "rugby ball-like shape" which was bright silver and metallic-looking as he flew at an altitude of 34,000 feet near Heathrow airport.

Although the pilot and airline have not been named, the plane involved was an Airbus A320 and the incident took place at 18:35 on 13 July 2013 some 20 miles west of the airport over Berkshire, where two other unexplained UFOs were pictured one month earlier (14 June).

Experts believe that due to the plane's altitude it probably wasn't taking off or landing from Heathrow but was probably travelling between another UK airport and the continent.

According to the head of the National Air Traffic Control Services, staff report an average of one unexplained "near-miss" with unidentified flying objects, or UFOs, every month. Like all near-misses in UK airspace, the incident was investigated by the UK Airprox Board. In their report they described the fear of the pilot who believed he was about to have a head-on collision with the object, which approached at great speed.

"He was under the apprehension that they were on collision course with no time to react. His immediate reaction was to duck to the right and reach over to alert the FO (First Officer); there was no time to talk to alert him. The captain was fully expecting to experience some kind of impact with a conflicting aircraft."

The pilot estimated that the UFO avoided the plane by just a few feet, streaking over the top of the cockpit. When the pilot contacted air traffic control they could offer no explanation as no other aircraft was in the area at the time. Meteorological balloons, toy balloons and military craft were also ruled out and the sighting remains unexplained.

Details of the incident have only now come to light, as the report has been published concluding that it was "not possible to trace the object or determine the likely cause of the sighting".

The Ministry of Defence closed its UFO desk and hotline in 2009, but unexplained sightings continue to be investigated by the Civil Aviation Authority because of the potential implications for flight safety.

Newly released archives from the Ministry of Defence and other government departments detail many similar incidents over the years. In one particularly puzzling case in 1991, both pilots of a passenger jet flying over Kent on its way to Heathrow saw a strange object whose presence was confirmed by the area control centre (ACC) operator on the ground. The incident was never explained.