As close encounters go, a woman's claim that an alien attempted to seduce her on a country road sounds far-fetched, but files just released reveal it was considered serious enough for the British Ministry of Defence to investigate.

The Norfolk woman who claimed she was approached by a man who said he came from another planet similar to Earth was questioned by the MoD intelligence branch DI55, whose brief was to investigate credible UFO reports.

She told them that, during their 10-minute chat, the man said his race was responsible for creating crop circles and explained the importance of contact between humans and his own people.

The unidentified woman in the Government's "X-Files" was described by officials as "agitated" following the incident in which she "heard a loud buzzing noise behind her, then turned to witness a large, glowing spherical object rise steadily until it disappeared".

The claims were examined by DI55 (whose very existence was denied by the Government until recently) and described by an intelligence official as "one of our most unusual UFO reports".

Other cases include the description of a black inverted boomerang-shaped UFO by two air traffic controllers at Heathrow. The 1992 sighting from the airport's control tower came a week after numerous witnesses in Louth, Lincolnshire, reported seeing three lights attached to a large, triangular craft.

The MoD documents also shed light on one of Britain's most infamous UFO episodes, the death of an American Air Force pilot, Captain William Schaffner, who conspiracy theorists believe was killed during a high-speed duel with aliens above the North Sea. In 1970 Schaffner's RAF Lightning crashed into the sea during a low-level exercise following take-off from RAF Binbrook, Lincolnshire. His body was never found, which became significant after claims his plane was scrambled to intercept UFOs.

The MoD files contain a summary of the RAF Board of Inquiry report into the crash, which makes no mention of UFOs. It concluded the pilot's death was a tragic accident.

Dr David Clarke, author of Flying Saucerers: A Social History of UFOlogy, said: "From suspected US Air Force spy planes to Russian rockets burning up in the atmosphere, these new files show the many and varied explanations for UFO reports submitted to the MoD. Making the material available allows us all to make an informed decision on the mystery of UFOs."