A student prank led to British Army and intelligence units being mobilised for fear that aliens were mounting an invasion of England, documents released today show.

Britain's National Archives has released previously classified files documenting sightings of unidentified flying object (UFOs) dating back to the 1950s.

One of the files reveals how the RAF was inundated with calls one morning in 1967 after residents of southern England awoke to find six small beeping UFOs lying in a perfect line from the Isle of Sheppey to the Bristol Channel.

A bomb disposal unit blew up one of the UFOs, another was airlifted to the south-eastern village of Aldermaston, and both the army and the Ministry of Defence's intelligence unit were mobilised for what was considered a real "alien invasion".

It was later discovered that engineering students at Farnborough Technical College had constructed and positioned the UFOs in a bid to raise money for charity.

The files contain around 8,500 pages which mainly cover the period from 1997 to 2005 and include photographs, drawings and descriptions of flying saucer sightings, as well as letters the MoD sent eyewitnesses in response to their accounts.

Policemen, a soldier, an RAF officer and members of the public reported sightings of objects including a "chewy mint-shaped solid craft" and aerial objects resembling a "ring", a "jellyfish" and a "silver voile spin top".

In one account, a man said he believed he had been "abducted" by aliens in October 1998 after seeing an unidentified craft hover over his London home and finding he had gained an hour of time in the process.

"It was a large cigar-shaped vehicle with big projectiles on each side like wings," he told the MoD.

"It seemed to have two very bright lights at the front and a white light flashing round and round underneath ... As you can imagine, I felt quite shaken."

The MoD wrote to the man informing him that the object was probably an airship, adding that the time he had gained was probably the result of the clocks being put back one hour on the night of his close encounter.

Invasion

"It does raise some questions about what really would happen if at any time in the future there was some kind of incident- would we be prepared?" said David Clarke, author of The UFO Files and consultant to the National Archives' UFO project.

One of the files documents the experiences of a retired RAF officer who said he saw a UFO while on holiday in Sri Lanka in April 2004 and sent the MoD supporting photographs.

"I noticed a partial aura in the sky, a minute or so later there was a clap of thunder, then a short while later a ring like a doughnut appeared," he told the MoD, adding that he thought it was an "air burst".

In its reply, the MoD suggested he contact the Sri Lankan government.

Other highlights include a UFO policy file from 1997 which reveals how the Ministry of Defence handled UFO reports and a file detailing the only full debate about UFOs ever to be held in Britain's House of Lords - in January 1979.

The MoD said it investigated every UFO sighting report it received to determine "whether there is any evidence that the United Kingdom's airspace might have been compromised by hostile or unauthorised air activity".

- Reuters