In 1985 a man wrote to the Ministry of Defence, politely explaining that he had been in contact with aliens since the age of 7. He had visited alien bases in Cheshire and Wirral, he said, befriended a charming alien called Algar and seen a UFO shot down next to Wallasey Town Hall, “as if done by an invisible entity”. The writer described how he had arranged for his alien chum to meet representatives of the Government, but sadly Algar was killed by rival aliens before the meeting could take place. The writer concluded: “That, of course, was that.”

The matter-of-fact tone is what makes this particular letter so touching, and so representative of the hundreds of similar alien and UFO sightings reported to the MoD and now made public for the first time.

The story of Algar is more dramatic than most UFO sightings. The vast majority of UFO spotters, as revealed in the MoD files, are not fantasists, but ordinary people who thought they saw something extraordinary in the sky. The spacecraft tend to come in familiar forms, with saucer and cigar shapes the perennial favourites. Coloured lights are also popular. (Why do aliens, who clearly have no wish to make contact, still tend to turn up here with all their lights flashing? Perhaps, like us, they just forget to turn the headlights off from time to time.)

Some UFO spotters have reason to be resentful, like the elderly Hampshire fisherman taken on a tour of a spaceship by little men in green overalls but then rejected as a captive human specimen because he was too old: a notorious example of space-ageism. Some are unwilling to believe their own eyes, like the Woking policemen who saw a white light descending in Horsell. (In H.G.Wells's War of the Worlds, the first Martians land on Horsell Common, which left the policemen wondering if their account would be mocked.) “Genuine report. Two competent officers slightly embarrassed,” notes the report.

A few sightings are genuinely compelling and intriguing, and some are sweetly humdrum: like the alien spacecraft tootling along the A839 to Lairg at a steady and law-abiding 30mph. But most are perfectly straightforward, each written up on a determinedly undramatic form by MoD bureaucrats: time, date, angle of flight, background of informant, etc.

“The sole interest of the MoD in UFO reports,” the ministry declares, “is to establish whether they reveal anything of defence interest.” They don't. Instead, they reveal something much more interesting: a passionate fascination with the mysteries of space that transcends gender, age, class and geography, and an urgent desire to believe in other worlds that amounts to a sort of secular spiritualism. Even the most sceptical reports come tinged with awe.

The sheer scale of the alien invasion is phenomenal. The British X-Files contain more then 7,000 separate sightings, 150 files in all, of which just eight have been released so far. The flight of the UFO is a tale deeply embedded in our cultural DNA, a part of modern folklore but also an ancient portent, reflecting the grip of the celestial unknown on human imagination down the ages. In the Old Testament, Ezekiel sees a fiery chariot in the sky. The most famous UFO of all hovered over Bethlehem.

The UFO story has evolved over time, but the fundamentals remain the same. The uncertainties of war and rapid technological advances tend to breed UFOs in large numbers. “What does all this stuff about flying saucers amount to?” Winston Churchill wondered in 1951. “What can it mean?”

Space travel brought other planets closer to earth, and an upsurge in alien visitation, but interest in UFOs has waxed and waned, often propelled by popular culture. The number of British UFO sightings almost doubled after the release of Close Encounters of the Third Kind in 1977.

Soon after, the MoD felt obliged to dismiss ufology as “claptrap”. Some psychologists believe alien abduction can be diagnosed as epilepsy or transient narcolepsy, a form of temporary sleep paralysis. UFO sightings have variously been put down to wishful thinking, credulity and too much beer. But at root, the phenomenon reflects something more profound: a sort of antidote to cosmic loneliness, the age-old urge to peer into the dark and wonder what or who might be out there.

You do not have to believe in flying saucers to believe in the flights of imagination they inspire. Many of the people reporting UFOs simply saw something unexpected and wondered, like Churchill, what it could mean.

We wonder still, but not like we used to. For the last decade UFO sightings have steadily declined. The British Flying Saucer Bureau closed in 2003. The number of UFOs dipped with the invention of the colour television and plunged with the advent of the internet. Perhaps in an complex and uncertain society, people have more practical concerns.

We no longer look into the sky and ponder the chances of anything coming from Mars. And even if we did, with the spread of light pollution, it would take a spaceship with very bright lights to be spotted at all.

The decline in UFO sightings may reflect a healthy scepticism, but a world without extraterrestrials would be drab indeed. The British X-Files reveal a people alert to the sky, imaginative, eccentric, slightly embarrassed and above all inquisitive. Perhaps this new proof of our former fascination with the mysteries of space will rekindle the curiosity. More likely, our remaining sense of wonder will erode still further; the flying objects of the future will be not only unidentified but unnoticed. And that, of course, will be that.