The Ministry of Defence set up a unit to look at UFO sightings in the 1950s but found no evidence of aliens, newly-released official documents have revealed.

The joint group was formed in 1951 after a spate of sightings in Sweden and the US sparked a "notable outbreak" of reports in the UK.

But the experts from the Directorate of Scientific Intelligence and the Joint Technical Intelligence Committee dismissed the prospect of alien arrivals.

They said sightings were "optical illusions and psychological delusions" - or just plain hoaxes.

Continuous watch

The group reported: "We consider that no progress will be made by attempting further investigation of unco-ordinated and subjective evidence.

"Positive results could only be obtained by organising throughout the country, or the world, continuous observation of the skies.

"[We would need] a co-ordinated network of visual observers, equipped with photographic apparatus, and supplemented by a network of radar stations and sound locators.

"We should regard this, on the evidence so far available, as a singularly profitless enterprise.

'Flat disc'

"We accordingly recommend very strongly that no further investigation of reported mysterious aerial phenomena be undertaken."

One of the cases they looked at was RAF Flight Lieutenant Hubbard who twice claimed to have seen "a flat disc, light pearl grey in colour, about 50ft in diameter" flying low over Farnborough at speeds to 800 to 1,000mph.

The scientists drily noted: "We find it impossible to believe that a most unconventional aircraft, of exceptional speed, could have travelled at no great altitude, in the middle of a fine summer morning, over a populous and air-minded district like Farnborough, without attracting the attention of more than one observer."

But conspiracy theorists sticking to the idea of a transatlantic cover-up of alien activity are likely to pick up on the fact some parts of the dossier are still withheld.