There's a UFO over Michael Howard's house...

Formerly secret files containing the details of hundreds of close encounters with possible extraterrestrial life were released by the Ministry of Defence (MoD) today.

But perhaps the most telling aspect of the records is their suggestion that the shapes of UFOs spotted in the skies over Britain have changed markedly over the past 50 years, suggesting people's sightings are influenced by what they see in television shows and films.

The files, with more than 6,000 pages of material gathered between 1994 and 2000, contain numerous reports of big, black, triangular aircraft with lights along the edges. In the 1940s and 1950s, the predominant form was saucer - or disc-shaped.

Dr David Clarke, an academic and author of The UFO Files, said: "In the 1950s, the next big leap in technology was thought to be a round craft that took off vertically and it's intriguing to note that this is the same period when people began to report seeing 'flying saucers' in the sky.

"In the period the latest file release covers, triangular-shaped US stealth bombers and Aurora spy planes featured heavily on TV, such as in The X-Files between 1993 and 2002, and films such as Independence Day, released in 1996, and the shape of reported UFOs corresponds."

Among the more interesting sightings is a large triangular UFO seen hovering above the home of the former home secretary and Conservative Party leader Michael Howard in March 1997. Eyewitnesses said they had seen a "humming" object the size of two passenger planes close to his house near Folkestone in Kent, but an RAF investigation found nothing unusual.

"It was just a huge triangle thing, which was a lot bigger than an aeroplane or anything like that," said one person who saw the object. "It had lights all around the outside, and this disc attached to the back, and a big light on the front. I pulled up to a stop and as I did it shot off." It is unclear whether the UFO influenced Mr Howard's decision to run for the Tory leadership in 2003.

A police officer also claimed he had seen something hovering over Chelsea FC's ground, Stamford Bridge, in south-west London on 10 March 1999. He described seeing an object with four bright yellow lights, square or diamond in shape, which hung in the air for 15 seconds before moving "across the sky fairly quickly, changing shape slowly".

The files also contain a memo written by the former prime minister Winston Churchill, in which he expresses his curiosity in "flying saucers" and requests a briefing on the subject. The memo, sent on 28 July 1952 to Lord Cherwell, the secretary of state for air, reads: "What does all this stuff about flying saucers amount to? What can it mean? What is the truth? Let me have a report at your convenience."

The response explained that UFOs could be explained by a combination of astronomical or meteorological phenomena; the mistaken identification of conventional aircraft, balloons or birds; optical illusions and psychological delusions; and deliberate hoaxes.

The files also show that some members of the public have struggled to persuade authorities their sightings were genuine. On 11 June 1997, a man claimed he had been visited by a "flying man" who lay on his quilt. The report said: "[The man] then called the police, who wouldn't come to his house because they thought he was a 'nutter'."

The files are the fifth instalment to be released under a three-year project between the MoD and the National Archives. For the next month, they can be downloaded for free through the National Archives' website.