LONDON (Reuters) - Hopes -- or fears -- that the Earth has been visited by alien life forms have been dismissed in an official report by British defense specialists.

The Ministry of Defense confirmed on Sunday a secret study completed in December 2000 had found no evidence that "flying saucers" or unidentified flying objects were anything other than natural phenomena.

The 400-page report, released under freedom of information laws to an academic from the northern city of Sheffield, 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.

"Evidence suggests that meteors and their well-known effects, and possibly some other less-known effects, are responsible for some unidentified aerial phenomena.

"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," it said.

A Ministry of Defense (MOD) spokesman said the full report, "Unidentified Aerial Phenomena in the UK Air Defense Region," would be published on its Web site on May 15.

The ministry publishes annual lists of UFO sightings on its Web site, which rank among its most viewed -- and bizarre -- pages. 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 defense 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."