An airline pilot's encounter with a UFO has left aviation authorities at a complete loss to explain it.
A "metallic cigar/rugby ball like" object passed within a few feet of his A320 Airbus near Heathrow airport when cruising at 34,000 feet.
The report into the incident states: "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."
After the pilot had regained his composure he reported it to air traffic control but by this point it has disappeared.
A report into the incident - which happened on July 13 last year - has only just been published.
It was compiled by the UK Airprox Board but does not provide an explanation for what it could have been.
It simply says it is "not possible to trace the object or determine the likely cause of the sighting".
Meteorological balloons and other aircraft have been ruled out.
Dr David Clarke, the UFO consultant for the National Archives, told the Telegraph: "This latest sighting is interesting, because it is detailed and clear. These pilots don’t file these reports for something and nothing.
"There was obviously something there."