Britain's defence ministry shut down its UFO unit four years ago after concluding that extra-terrestrials likely did not exist, and in any case did not pose a threat, previously secret files released Friday showed.

The Ministry of Defence (MoD) closed its hotline in 2009 despite a trebling of reported sightings of Unidentified Flying Objects (UFOs) that year, many of them near national landmarks.

In a briefing for then defence minister Bob Ainsworth, civil servant Carl Mantell said the UFO desk was using up increasing amounts of staff time but had "no valuable defence output".

He wrote in a memo 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".

It added: "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."

The National Archives files reveal details of sightings recorded in the two years before the UFO desk was disbanded, including those around the Houses of Parliament and Stonehenge.

Between 2000 to 2007, the MoD received an average of 150 reports a year, but 520 sightings were recorded in the 11 months to November 2009, according to a briefing in the files.

Officials said one possible reason for the surge could have been the trend for releasing Chinese lanterns, which appear like floating lights in the sky.

Many sightings were made in the summer months by people out walking their dogs, having barbecues and, in one case, relaxing in a hot tub.