Britain has released hundreds of previously secret UFO files, including a letter saying that Winston Churchill ordered a 50-year cover-up of a wartime encounter between a UFO and an RAF 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 Defence (MoD) 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 Mr Churchill and US General Dwight D Eisenhower during which the two expressed concern over the incident and "decided to keep it secret".

The MoD subsequently investigated the case but found no written record of the incident, the files say.

"The MoD 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," it said in a 1999 note.

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 MoD over past decades.

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

One memo, dated 1997, contains reports of "sonic booms" and a mysterious plane crash in northern England. No wreckage was found in an ensuing search by the police and rescue teams.

Another incident refers to sightings of a "black triangular UFO" over the home of the shadow home secretary in Kent in the late 1990s. An investigation showed no breach of security.

In a case filed in 1995, the captain of a plane approaching Manchester airport reported a near-miss with an "unidentified object", and a witness on the ground separately provided a sketch showing a UFO "20 times the size of a football field".

The memo said an inquiry failed to identify the object.

Cold War documents

Buried deep among meticulous sketches and MoD memos, some files refer to curious episodes in Britain's history.

During the Cold War, Britain sent fighter jets to intercept Soviet aircraft as often as 200 times a year, one MoD document showed.

The note, filed in 1996, said mystery sightings picked up on radar during the Cold War were invariably proved to be Soviet anti-submarine or long-range reconnaissance planes.

"Prior to the demise of the former Soviet Union, aircraft were scrambled some 200 times annually to intercept and investigate uncorrelated tracks penetrating the UK Air defence region (AKADR) from the north," the document said.

The last such scramble was in September 1991 - around the time of the collapse of the Soviet Union.

- Reuters