Files of hundreds of UFO sightings, including one where a man developed a skin condition after a supernatural encounter, have been released in Britain.

The most unusual of the newly declassified Ministry of Defence files tells how a man in south Wales, claimed his car was surrounded for five minutes by a "tube of light" in 1997.

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The man, who reported the incident to police, was sick the next day and also developed a mysterious skin condition.

"First sighting seemed like a massive star moving towards the car," the previously top secret police report said.

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"Having stopped the car and switched the lights off, the light encircled the car, remaining for perhaps five minutes.

"(The man) was able to walk through the light which was very bright. There was no sound, no aircraft or helicopter noise, and besides feeling ill (he) was very frightened."

The newly released files span 6000 pages and cover the period 1994 to 2000.

They were released alongside another file dating back to 1952 which showed then British prime minister Winston Churchill requesting a briefing on UFOs from his secretary of state for air.

"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," he wrote.

In the reply, Churchill was told that alleged UFO sightings could be explained by earthly phenomena such as optical illusions, mistaken identification of planes, birds and balloons and deliberate hoaxes.

The more recent files detailed reams of other UFO spottings.

A policeman said he saw one flying over Chelsea Football Club's Stamford Bridge ground in southwest London in 1999, while a man in Birmingham, central England, saw a blue triangle hovering over his garden late one night in 1997.

That alleged alien craft flew off quickly, leaving behind a "silky-white" substance on nearby treetops which the man collected in a jam jar.

One UFOlogist, David Clarke of Sheffield Hallam University in northern England, said the latest files showed how the shape of reported flying saucers had changed in recent decades.

This could be linked to sci-fi television shows and films like "The X Files" and "Independence Day", he said.

"One interpretation could be that the latest advances in technology may be influencing what people see in the sky," he said.