In spite of the secrecy surrounding the UFO study, it seems citizens of planet Earth have little to worry about.

The report, which was completed in 2000 and stamped "Secret: UK Eyes Only", has been made public for the first time.

Only a small number of copies were produced and the identity of the man who wrote it has been protected.

His findings were only made public thanks to the Freedom of Information Act, after a request by Sheffield Hallam University academic Dr David Clarke.

The four-year study - entitled Unidentified Aerial Phenomena in the UK - tackles the long-running question by UFO-spotters: "Is anyone out there?"

The answer, it seems, is "no".

Never in thirty years of working out of doors at all hours have I seen anything that cannot be explained by a few seconds reasoned thought   Barry P, Havant, UK

It adds: "There is no evidence that 'solid' objects exist which could cause a collision hazard."

So if there are no such things as little green men in spaceships or flying saucers, why have so many people reported seeing them?

Well, here is the science bit.

"Evidence suggests that meteors and their well-known effects and, possibly some other less-known effects are responsible for some unidentified aerial phenomena," concludes the report.

Meteors may have been responsible for some UFO sightings

"They appear to originate due to more than one set of weather and electrically charged conditions, and are observed so infrequently as to make them unique to the majority of observers."

People who claim to have had a "close encounter" are often difficult to persuade that they did not really see what they thought they saw. The report offers a possible medical explanation.

"The close proximity of plasma related fields can adversely affect a vehicle or person," states the report.

"Local fields of this type have been medically proven to cause responses in the temporal lobes of the human brain. These result in the observer sustaining (and later describing and retaining) his or her own vivid, but mainly incorrect, description of what is experienced."

There are, of course, other causes of UFOs - aeroplanes with particularly bright lights, stray odd-shaped balloons and strange flocks of birds, to name but a few.

The report admits its findings will not persuade everyone

Some UFO-spotters believe governments will always cover up the truth about UFOs, because they are afraid of admitting that there is something beyond their control.