Although many of the sightings can be dismissed as false alarms, a minority elude proper explaination.

"Most of the UFO sightings here are probably misidentifications of aircraft lights and meteors," said Nick Pope, the man responsible for investigating the sightings at the MoD.

"But some are more difficult to explain, and include UFOs seen by police officers and pilots, and cases where UFOs have been tracked on radar."

Folklore expert Dr David Clarke of Sheffield Hallam University has been assisting the National Archives in the release of the documents.

As one of the few academics to take UFOs seriously, Dr Clarke has in-depth knowledge of UFO encounters that cannot be dismissed easily.

One of the reports in the archives was filed from an anonymous small airport near the east coast of England.

Experienced air traffic controllers described how they were guiding a small plane coming in to land, when they saw a brightly lit object approach another runway without clearance.

"Everyone became aware that the object was unidentified," the report reads.

"SATCO [code name for a controller with 14 years experience] reports that the object came in 'at speed,' made a touch and go on runway 27, then departed at 'terrific speed' in a 'near vertical' climb."

"They were absolutely astonished," Dr Clarke said.

"It was a bright, circular object, flashing different colors, and after it touched down it disappeared at fantastic speed."

Dr Clarke is convinced that the report should be taken seriously as the men involved were air traffic controllers with eight years experience on the job.

"The report comes from very qualified people, and it's one of the few that remained unexplained."

The MoD has always maintained that there has never been any contact with aliens, despite a plethora of conspiracy theories to the contrary.

A ministry memo from 1983 says: "The sole interest of the Ministry of Defence in UFO reports is to establish whether they reveal anything of defence interest (e.g. intruding aircraft)."

"The Ministry of Defence does not deny that there are strange things to see in the sky."

"It certainly has no evidence that alien spacecraft have landed on this planet."

Having examined the documents in the archives, Dr Clarke can find no physical evidence for alien contact.

"The Ministry of Defense doesn't have any evidence that our defences were breached by alien craft," he said.

"They never found one, no bits of one."

"That's all we can say."