The captain of the Alitalia airliner was so concerned he shouted "look out" to his co-pilot after seeing a brown missile-shaped object shoot past them overhead.

The mysterious incident near Lydd in Kent in 1991 was thoroughly investigated by the Civil Aviation Authority (CAA) and the military.

But having ruled out the object being a missile, weather balloon or space rocket, the MoD closed the inquiry and left the matter unsolved.

The unexplained close encounter is one of many recounted in military UFO documents made available online today by the National Archives.

Covering sightings from between 1986 and 1992, the 19 files include:

1) a US Air Force pilot's account of being ordered to shoot down a UFO that appeared on his radar while he flew over East Anglia;

2) an MoD request that Army and Navy helicopters should not take photographs of crop circles for fear of undermining the official line that the military did not investigate unexplained phenomena;

3) a letter from a woman claiming to be from the Sirius system who said her spacecraft - also containing two "Spectrans" with "Mr Spock ears" - crashed in Britain during the Second World War.

But one of the most intriguing episodes is the near-miss involving the Alitalia airliner at about 8pm on April 21 1991.

The McDonnell Douglas MD80 aircraft was en route from Milan to Heathrow at 22,000ft with 57 people on board when pilot Achille Zaghetti saw the strange object some 1,000ft above him.

He recounted: "At once I said, 'look out, look out,' to my co-pilot, who looked out and saw what I had seen.

"As soon as the object crossed us I asked to the ACC (area control centre) operator if he saw something on his screen and he answered 'I see an unknown target 10nm (nautical miles) behind you'."

A CAA document notes that Southern TV broadcast a story about a 14-year-old boy who reported seeing a missile flying at low level before climbing through the cloud and disappearing on the same evening.

Radar images showing the UFO were initial labelled "cruise missile??" but it was quickly established that it was not a military weapon.

By July 2 the MoD had concluded that the object had not come from the Army firing ranges in the Lydd area and that there was no known "space-related activity" that night.

An unnamed Whitehall official wrote: "It is our intention to treat this sighting like that of any other Unidentified Flying Object and therefore we will not be undertaking any further investigation."

There were, however, a number of other similar incidents recorded the same year.

On June 17 1991 four passengers on board a Dan Air Boeing 737 saw a "wingless projectile" pass beneath the aircraft as it climbed from Gatwick Airport headed for Hamburg.

Gatwick air traffic control were alerted but said they were "unaware of anything unusual occurring at that time".

The MoD said a weather balloon was released in the area at around this time, but it noted that the description and timing did not match.

Then on July 15 the pilot of a Britannia Airways 737 reported seeing a "small black lozenge-shaped object" travelling at speed as they approached Gatwick.

An investigation found that the object "did not seem to fit any recognisable piece of aviation equipment" and failed to reach a firm conclusion.

Dr David Clarke, a UFO expert and journalism lecturer at Sheffield Hallam University, said the documents would shed new light on relatively little-known sightings.

He admitted that some conspiracy theorists will already have decided that the release of the papers is a "whitewash".

"Because the subject is bedevilled by charlatans and lunatics, it is career suicide to have your name associated with UFOs, which is a real pity," he said.

"The National Archives are doing a fantastic job here. Everyone brings their own interpretation. Now you can look at the actual primary material - the stuff coming into the MoD every day - and make your own mind up."

:: The files can be accessed at www.nationalarchives.gov.uk/ufos