The Ministry of Defence launched an investigation into a flurry of UFO sightings across the West in the late Eighties and early Nineties, according to secret papers released yesterday.

Incidents included bizarre spacecraft making crop circles, mysterious burning lights seen for miles and triangular shapes flying over two West cities.

But anyone hoping for Government acknowledgement of extra-terrestrial life visiting Britain in the release of the files under the National Archives' 15-year-old rule is going to be disappointed.

The MoD and official UFO investigators pour scorn on the sightings, dismissing them as misidentified pop festival light shows, American military jets refuelling and hot air balloons.

The period covered by the latest publications deals with the crop circle mania between 1987 and 1993, when the fields of Wiltshire were full of researchers, UFOlogists and worldwide media.

In the early days of the crop circle phenomenon, the MoD explained crop circles were a naturally occurring phenomenon, but when it became apparent in the late Eighties that they weren't, the military changed tack.

In 1989, at the height of crop circle mania, the secret papers reveal that when Mrs Thatcher expressed curiosity over the phenomenon and interest in the work of the early crop circle researchers, the semi-official Government UFO researchers wrote to her to warn her not to be taken in.

The British UFO Research Association, which had close links with the MoD, told Mrs Thatcher: "We have been studying the circles since they first appeared in 1980 and can provide documented evidence that demonstrates just how much of what Andrews and Delgado (the two leading croppies at the time) are claiming is highly suspect.

"These circles appear throughout Britain, have done so for many years, and are no threat to anybody."

That line became official MoD policy on crop circles, but when multiple circles and formations appeared, the MoD was forced to change tack, and blamed people.

One farmer near Avebury complained of crop circles appearing in his fields, and blamed helicopters from nearby RAF Odiham.

The circles were investigated, and one RAF chief wrote to MoD chiefs: "He believed that the damage may have been caused by hovering helicopters and asked us to take a look.

"This was definitely not the case, and I personally suspect hippies with scythes!"

The MoD couldn't explain one report which will excite modern-day crop circle researchers.

In July 1991, a driver and his wife pulled off the A350 south of Warminster and watched, spellbound, as a bright whirlwind created a crop circle in front of them.

The MoD report read: "Four whirlwinds, spiral from centre of circle – bright white.

"Swishing noise (distant). Moving around each other in a form of a circle and then disappeared."

The released papers also detail mass sightings by hundreds of people of the same mysterious phenomena often miles apart.

On August 4, 1987, hundreds of people reported seeing strange triangular-shaped objects and lights travelling over Devon.

First sighted in Exeter and then Plymouth, the MoD later told witnesses the lights were probably from US military jets refuelling.

The papers also reveal how people from all over Britain, including the West Country, sent in sketches of UFOs they saw, and even the aliens who they claimed emerged from them.