Military sightings of UFOs, an "alien abduction" in London and an unidentified aircraft shadowing a Lancaster bomber feature among thousands of close-encounter documents released by the National Archives today.
The extra-terrestrial files reveal how the phenomenon was discussed at the highest level of government and security services worldwide, including at the United Nations (UN), the US Central Intelligence Agency (CIA) and was even the subject of a debate in the House of Lords.
The previously-classified records show that in January 1979 - during the peak of the "Winter of Discontent" - in addition to discussions on trade union strikes, the House of Lords held a debate of the subject of UFOs - the only full debate on UFOs ever held in British Parliament.
The files reveal that in December 1977 the government used its influence to talk down a call by Grenada president, Sir Eric Gairy, for a UN agency to conduct research into UFO sightings.
Gairy eventually withdrew his proposal but continued his campaign for a full UN debate on UFOs - calling on the UN General Assembly to make 1978 "the year of the UFO".
One of the 35 newly-released files shows 15 unidentified aircraft were detected on radar approaching the UK between January and July 2001 in the months leading up to 9/11.
The Ministry of Defence (MoD) received just one UFO report (with no radar corroboration) on September 11 itself.
Other highlights include:
* Claims that the Home Office had emergency procedures for dealing with landed and crashed satellites and UFOs.
* Details on RAF interception policy during the Cold War - when aircraft were scrambled on a daily basis to intercept Warsaw Pact aircraft approaching the UK coast.
* US policy files on UFOs, including CIA papers discussing the use of UFO reports for "psychological warfare".
* An alleged UFO sighting by crew of HMS Manchester off the coast of Norway and how the logbook recording the incident could not be recovered.
* A man who believed he may have been abducted by aliens after seeing an unusual aircraft one evening and experiencing a period of missing time.
* One report describing a "War of the Worlds" incident in 1967 that, for a few hours at least, was treated as a potentially real "alien invasion" of the UK.
The RAF were flooded with calls from the public reporting six small "flying saucers" discovered in locations in a perfect line across southern England from the Isle of Sheppey to the Bristol Channel.
Four police forces, bomb disposal units, the army and the MoD's intelligence branch were all mobilised before it emerged the saucers were a 'rag-day' hoax by engineering students from Farnborough Technical College.
The tranche of files also detail in full the freedom of information requests and letters from "persistent enquirers" that led to the MoD opening the UFO files for the first time in history.
Dr David Clarke, author of the book The UFO Files and senior lecturer in journalism at Sheffield Hallam University, said: "Before the Freedom of Information (FOI) Act, we had to wait 30 years or more before we could see files on UFOs.
"Following its introduction, questions on UFOs ranked in the top-three most popular FOI requests received by the Ministry of Defence.
"I was one of the MoD's most 'persistent correspondents' and eventually persuaded MoD and other government departments to release their information on this perplexing and controversial subject.
"You can see from the files that I wasn't the only one interested in the subject, with the phenomenon discussed at the highest level of government right across the globe."
Today's release is the largest disclosure of documents so far by The National Archives.
The files contain over 8,500 pages of UFO sightings and reports, colour photographs and drawings, RAF investigations, unusual radar detections, parliamentary briefings and - for the first time - documents on the government's policy on UFOs.
The policy files show the discussions and correspondence that led to a change in government policy on UFOs and the formation of a UFO sighting hotline.
Details in the files show how the workload of the UFO desk at the MoD increased by 50% during 1996/97, due to media interest in the subject around the 50th anniversary of the Roswell incident.
The files are available to download for free for a month from the website: http://www.nationalarchives.gov.uk/ufos