The documents refer to US policy files on UFOs, including CIA papers discussing the use of such reports for "psychological warfare", and contain claims that the British government had emergency procedures for dealing with crashed satellites and UFOs.
UK's X files expose the unexplained and the absurd
LONDON // Britain released its X files on Thursday - more than 8,000 pages of reports of UFO sightings and international concerns over the possibility of a close encounter of the third kind.
Ranging from the inexplicable to the absurd, the extra-terrestrial files released by the UK's National Archives illustrate how incidents were discussed at the highest level of government and by security services worldwide, including at the UN and CIA.
The previously classified reports show, for instance, that in 2003, governments internationally discussed a sighting in south-east England when a family captured on film red and white lights zigzagging their way across the sky.
Several police officers, including a helicopter crew, also saw the lights although radar checks revealed nothing unusual in the area.
"A policeman sent to investigate confirmed the sighting," the report reads. "Objects were moving faster than any man-made aircraft."
The files also show that, in 1967, Britain went on a "War of the Worlds" footing for several hours after a half-dozen, beeping and apparently seamless silver saucers were found in a perfect line across southern England.
Calls from the public sparked the mobilisation of army and bomb disposal units, and caused four police forces and the defence ministry's intelligence unit to go on full alert (although a village copper who found one of the saucers in the West Country, simply tucked it under his arm and cycled back to his police station).
In the end, this alien invasion turned out to be a prank, with the craft having been built by engineering students at a technical college in Surrey.
More mysterious were three circular objects reported by an airline pilot - described in the files as "a credible witness" - over East Sussex and the report from an RAF crew in 1990, who were overtaken by an object "the size of a C130 Hercules" as they flew at almost the speed of sound over the North Sea.
A possible explanation was offered by defence officials who said it could have been a Russian rocket booster re-entering the earth's atmosphere.
David Clarke, a university lecturer and author of the book The UFO Files, has been campaigning for several years to have the files "on this perplexing and controversial subject" released under Britain's Freedom of Information Act.
"I'm interested in why people believe in these things - regardless of proof," he told the BBC. "In hundreds of years' time, people will look back at these files and draw conclusions about the kind of people we were and the things we wanted to believe in."
The files - which will be free to download for the next month at www.nationalarchives.gov.uk/ufos - also contain photographs and drawings of phenomena.
For the conspiracy theorists, there is the inexplicable disappearance of intelligence reports compiled after the "Rendlesham incident" in 1980 when US servicemen based in Suffolk reported seeing mysterious lights in the sky.
Official reports speak of a "huge hole" in the files but are unable to explain what happened to the intelligence reports.
The documents also refer to US policy files on UFOs, including CIA papers discussing the use of such reports for "psychological warfare", and contain claims that the British government had emergency procedures for dealing with crashed satellites and UFOs.
Files also show that, in 1977, the government used its influence to prevent Sir Eric Gairy, then president of Grenada, from getting a UN agency to formally investigate UFO sightings around the world.
Such an investigation would have probably dismissed the sighting by a man in south-west London in October, 1998, who, late at night as he stood in his garden with a drink, reported "a large, cigar-shaped vehicle with large projectiles on each side like wings" roaring over his house.
Worse, the following morning, as he listened to the nine o'clock news on radio, he noticed his watch said 10 o'clock. "I am now beginning to wonder if I was abducted," he reported.
Very patiently, a ministry official wrote back that the UFO was probably a plane landing at nearby Heathrow and that the "abduction" could be explained by the fact that, that night, the nation's clocks went back an hour in the move from British Summer Time to GMT.