The Ministry of Defence closed its UFO desk set up to find evidence of extra-terrestrial visitors - because there was simply no evidence of extra-terrestrial visitors.
The desk was wrapped up in 2009 with officials citing it served "no defence purpose" and was taking staff away from "more valuable defence-related activities", newly released files showed on Thursday.
The latest tranche of declassified MoD UFO files showed the decision was taken to close the desk and its UFO "hotline" in a year when sightings reported to the department had trebled, but that, in more than 50 years, none had indicated the existence of "any military threat to the UK".
According to a briefing in the files, during the years 2000-07 the MoD received an average of 150 reports per year.
But by November 2009, it had already received 520 reports that year, as well as 97 freedom of information requests on UFOs.
Possible reasons for the increase included the craze for releasing Chinese lanterns at weddings and public holidays, appearing as floating lights in the sky.
One example included a sighting by soldiers in Shropshire which hit the headlines in June 2008.
A memo on the incident, referring to the sighting over Tern Hill Barracks in Shropshire on 7 June 2008, which had been reported in The Sun newspaper, described how a group of soldiers had seen lights in the sky and made a video which they passed to the tabloid.
In the file on the sighting, an unnamed official, who had obtained a copy of the video, described how it showed a number of lights in the sky, but their change of colour and square appearance seemed to be due to the photographer zooming in.
Photograph apparently showing a 'UFO' by Stonehenge, Wiltshire
"The BBC has reported that at the same time as the alleged incident, a local hotel was letting off Chinese lanterns and, apparently, the hotel manager thinks the whole UFO story is 'highly hilarious'," the official wrote.
"I do not intend to investigate any further as I think we have our answer... "
Story continues after the slideshow...
The 25 files released by the National Archives include 4,400 pages and cover the work carried out in the final two years of the MoD's UFO desk, from late 2007 until November 2009.
They include accounts of alleged abductions and contact with aliens and UFO sightings near UK landmarks, as well as documentation of the decision to close the UFO desk.
The decision was taken to close the desk and bring to an end the "UFO hotline" by officials who deemed it had no "defence benefit", and the resources that were being devoted to it were taking staff away from "more valuable defence-related activities".
In a briefing for then defence minister Bob Ainsworth in November 2009, Carl Mantell, of the RAF's Air Command, suggested the MoD should try to significantly reduce the UFO task, "which is consuming increasing resource, but produces no valuable defence output".
He told Mr Ainsworth that, in more than 50 years, "no UFO sighting reported to (MoD) has ever revealed anything to suggest an extra-terrestrial presence or military threat to the UK".
The memo said there was "no defence benefit" in the recording, collating, analysis or investigation of the sightings, adding: "The level of resources diverted to this task is increasing in response to a recent upsurge in reported sightings, diverting staff from more valuable defence-related activities."
Officials predicted a backlash from "ufologists" to the decision to close the UFO desk, and also noted that they had "deliberately avoided formal approaches to other governments on the issue", amid fears of "international collaboration and conspiracy".
The files also revealed campaigns by ufologists for the government to investigate sightings more thoroughly, with letters sent to senior ministers, former prime minister Gordon Brown, and even the Queen, calling for more action.
After the closure, air traffic control centres and local police forces were advised to no longer refer UFO sightings to the MoD.
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An official MoD statement said: "The Ministry of Defence has no opinion on the existence or otherwise of extra-terrestrial life.
"However, in over 50 years, no UFO report has revealed any evidence of a potential threat to the United Kingdom.
"The MoD has no specific capability for identifying the nature of such sightings. There is no defence benefit in such investigation and it would be an inappropriate use of defence resources.
"Furthermore, responding to reported UFO sightings diverts MoD resources from tasks that are relevant to defence.
"Accordingly, and in order to make best use of defence resources, we have decided that from 1 December 2009 the dedicated UFO hotline answer-phone service and email address will be withdrawn. MoD will no longer respond to reported UFO sightings or investigate them."
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Dr David Clarke, author of the book The UFO Files, said: "The last pieces of the puzzle have finally been revealed with this insight into the last days of the UFO desk.
"These files spell out clearly why the MoD decided - after 60 years - it no longer needed to keep tabs on sightings, even those made by 'credible' people such as police officers and pilots.
"The last files from the UFO desk are now all in the public domain.
"People at home can read them and draw their own conclusions about whether 'the truth' is in these files or still out there."
Sightings recorded in the 25 MoD files released today, which cover the years 2007 to 2009, include:
- A letter from a school child to the MoD asking for the truth about UFOs after she had seen some strange lights, and including a drawing of an alien in a UFO waving.
- A report received via the UFO hotline by someone who had been "living with an alien" in Carlisle for some time and one from a man from Cardiff who claimed a UFO abducted his dog, car and tent while he was camping with friends in 2007.
- Sightings of UFOs over the Houses of Parliament, Stonehenge, and Blackpool Pier.
The files are available to download for free for a month from the website: www.nationalarchives.gov.uk/ufos
They can also be viewed using a new app, UFO Files UK, created by Black Plaques in association with the National Archives.