England ufo
© UK National ArchivesA sketch from the National Archives' UFO files shows a "flying saucer" that was purportedly sighted over a cornfield at East Grinstead in 1995.
Britain released hundreds of previously secret "UFO files" on Thursday, including a letter saying that Winston Churchill had ordered a 50-year cover-up of a wartime encounter between an unidentified flying object and military pilot.

The files, published by the National Archives, span decades and contain scores of witness accounts, sketches and classified briefing notes documenting mysterious sightings across Britain.

One Ministry of Defense note refers to a 1999 letter stating that a Royal Air Force plane returning from a mission in Europe during World War II was "approached by a metallic UFO."

The unidentified author of the letter says his grandfather attended a wartime meeting between Churchill and Dwight Eisenhower during which the two expressed concern over the incident and "decided to keep it secret."

The Ministry of Defense subsequently investigated the case but found no written record of the incident, the files say. In a 1999 note, the ministry said it "does not have any expertise or role in respect of 'UFO/flying saucer' matters or to the question of the existence or otherwise of extraterrestrial lifeforms, about which it remains totally open-minded."

Latest in a series of releases

Britain has been slowly releasing long-classified files related to sightings of mysterious craft in the skies above its cities, compiled and investigated by the Ministry of Defense over past decades.

Some cases subsequently received rational explanations, such as meteors burning up in the atmosphere, but many are unsolved.