The British ministry of defence confirmed on Sunday that a top secret study completed in December 2000 found no evidence that unidentified flying objects were anything other than natural phenomena.
The 400-page report was released under the freedom of information act to an academic from Sheffield and concluded that meteors and unusual atmospheric conditions could explain UFO sightings such as bright lights in the sky.
"No evidence exists to suggest that the phenomena seen are hostile or under any type of control, other than that of natural physical forces," the report said, according to extracts quoted by the BBC.
"Considerable evidence exists to support the thesis that the events are almost certainly attributable to physical, electrical and magnetic phenomena in the atmosphere, mesosphere and ionosphere."
A ministry of defence (MOD) spokesman said the full report, Unidentified Aerial Phenomena in the UK Air Defence Region, would be published on its website on May 15.
"Considerable evidence exists to support the thesis that the events are almost certainly attributable to physical, electrical and magnetic phenomena"
British ministry of defence
The annual lists of UFO sightings the ministry publishes are among the most visited pages on its site.
In 2005, the ministry was asked under freedom of information laws for details of its plans for "dealing with the arrival of extra-terrestrials".
An unnamed defence official replied: "While we remain open-minded, to date the MOD knows of no evidence which substantiates the existence of these alleged phenomena and therefore has no plans for dealing with such a situation."