UFO documents to be released by the Ministry of Defense covers the period from 1987 to 1993.
UFO files show 'close encounter'
A boomerang-shaped object seen from an airport control tower and a woman's encounter with an "alien" are among the secrets revealed in official UFO files.
In November 1989 a "completely terrified" woman contacted RAF Wattisham in Suffolk to report her close encounter with a man claiming to be an alien.
She said she met the fair-haired man with a Scandinavian-type accent as she walked her dog on a sports field. He told her crop circles were caused by others like him who had travelled to Earth and that the purpose of his visit was friendly.
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The files, which can be downloaded from the National Archives website, are being released as part of a three-year project.
These latest documents are the first containing information written by defence intelligence staff to be made public.
Dr Clarke said they were among tens of thousands of secret files contaminated with asbestos and were in danger of being destroyed.
They were eventually saved after a campaign by historians to rescue them from the old War Office building in Whitehall.