Sketch of a UFO spotted in Hemsworth, West Yorkshire, on 25 March 1988
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.
The woman reported seeing a glowing, spherical object rise into the air in Norwich after meeting a man who said he came from a planet similar to Earth.
In another sighting, a triangular craft hovered then "shot off at 500mph".
The third set of UFO documents to be released by the Ministry of Defence covers the period from 1987 to 1993.
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.
He then said he had spoken to her because he felt it was important to have contact with humans even though he was told not to.
As the unidentified woman ran home she heard a loud buzzing noise and turned to see a large, spherical object, glowing orange-white, rise steadily until out of sight.
A letter from RAF Wattisham to the MoD and Norfolk police described it as "one of our more unusual UFO reports".
Three years later two air traffic controllers at London's Heathrow Airport reported seeing a black, inverted boomerang-shaped UFO from their control tower.
It was stationary then moved steadily in the morning sun, the files say.
A man's drawing of UFOs spotted in Uttoxeter, Staffordshire, in 1987
A week earlier on a coastal road in Louth, Lincolnshire, several people had reported similar sightings of a large, triangular-shaped object with three lights.
Many witnesses reportedly stopped their cars and got out for a better look before it turned on its axis and zoomed off "at 500mph towards north west".
Other accounts of strange lights and unexplained objects had plausible explanations.
In November 1990, the crews of six RAF Tornado jets reported being overtaken by a "giant UFO" while flying over Germany.
They thought it was a test flight for the then top-secret US Stealth fighter, but it turned out to be the burning debris from a Soviet rocket body used to launch a satellite into orbit.
On 31 March 1993 various reports of moving lights over south-west England and south Wales were later traced to the re-entry of a Russian Cosmos rocket body.
The files suggest it had burned up, disintegrating over the North Atlantic.
In 1992, a bright cigar-shaped object seen flying silently over central London at night triggered a "spate" of sightings, notably in Ilford and Romford.
It was later identified "almost certainly" as an illuminated airship advertising the Ford Mondeo car.
UFO expert Dr David Clarke said: "The vast majority of reports are ordinary things seen in extraordinary situations.
"So many things can be interpreted as unusual, you've got to eliminate all that noise and see what's left.
"I don't think there's any solid evidence that we have been visited by intelligent life but I don't think you can rule that out.
"There are many good examples of puzzling things, for example seen on radar by the military, that need investigating."
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.
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