The Ministry of Defence closed its UFO desk because it served "no defence purpose" and was taking staff away from "more valuable defence-related activities", newly released files show.
The desk was closed in December 2009 despite a surge in reported sightings.
The disclosure came in National Archives files relating to reports of UFOs - Unidentified Flying Objects - between 2007 and November 2009.
They show UFOs were reported at several UK landmarks, including Stonehenge.
The latest tranche of declassified files covers the final two years of work carried out by the MoD's UFO desk.
The 25 files include reports alleging contact with aliens and UFO sightings near UK landmarks, and detail the decision to close the MoD's dedicated desk and "hotline".
In a briefing for the then defence minister, Bob Ainsworth, in November 2009 a civil servant, Carl Mantell of the RAF's Air Command, suggested the MoD should try to significantly reduce the UFO work. He said it was "consuming increasing resource, but produces no valuable defence output".
He told Mr Ainsworth that in more than 50 years, "no UFO sighting reported to [the MoD] has ever revealed anything to suggest an extra-terrestrial presence or military threat to the UK".
His 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."
An official MoD statement from the time said the department had "no opinion on the existence or otherwise of extra-terrestrial life".
It went on: "In order to make best use of defence resources, we have decided that from December 1 2009 the dedicated UFO hotline answer-phone service and email address will be withdrawn. [The] MoD will no longer respond to reported UFO sightings or investigate them."
'Living with an alien'
Among the 4,400 pages of documents released are:
- A letter from a school child in Altrincham, Greater Manchester, to the MoD, dated January 2009, asking if aliens exist 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
- A report from a man from Cardiff who claimed a UFO abducted his dog, and took his car and tent, while he was camping with friends in 2007
- "Green, red and white lights" reportedly seen over the Houses of Parliament in London in February 2008
- "Discoid" shapes in photographs of Stonehenge, in an email dated in January 2009
- Photographs taken at Blackpool Pier which show an aircraft that had not been seen at the time the picture was taken in October 2008
The files show the number of UFO sightings reported to the MoD trebled in the year the desk was closed.
According to a briefing in the files, during the years 2000-07 the ministry 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 rising popularity for releasing Chinese lanterns during celebrations.
Dr David Clarke, author of the book The UFO Files, said the "last pieces of the puzzle" had been revealed with the insight into the final days of the UFO desk.
"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," he said.