The truth is out there... and will be coming to a computer near you in the next few weeks as the Ministry of Defence releases its UFO files on the internet.

The decision to provide details of more than 15,000 alleged sightings of flying saucers and other unidentified flying objects over Britain since 1959 has been taken to reduce the costs associated with Freedom of Information inquiries by enthusiasts and conspiracy theorists.

The sightings were reported from John O' Groats to Land's End by RAF and civilian pilots, air-traffic controllers, policemen and MoD employees as well as the public.

They were examined by a secret section of the directorate of air staff "to establish whether the UK's airspace might have been compromised by unauthorised air activity", a spokesman said.

The most common witness accounts included reports of orange discs, spheres, triangles and balls of fire which could change formation. An undisclosed but "considerable" number were seen over RAF and US Air Force bases in England.

The official policy line is "the MoD does not have any expertise or role in respect of UFOs or flying saucer matters or to the existence or otherwise of extraterrestrial lifeforms, about which it remains totally open-minded". The spokesman added: "We know of no evidence which substantiates the existence of such phenomena. Unless there is evidence of a potential threat to the UK from an external source and, so far, no report has revealed such evidence, we do not investigate the precise nature of each sighting.

"We believe rational explanations such as aircraft lights or natural phenomena could be found if resources were diverted for this purpose. But it is not the function of the MoD to provide this kind of aerial identification service."

The figures show the peak years for sightings were 1978, with 750 reports, and 1999, with 609.

Files published under the FoI Act show the village of West Kilbride in Ayrshire remains the UK's unlikely leading hot spot for sightings and has displaced Bonnybridge, near Falkirk, as Scotland's "flying saucer capital". There were 12 separate reports of "yellow spheres" flying alone or in formation above the village in 2005. All were referred to the air directorate.

The MoD abandoned inquiries into the UFO phenomenon in 1951 but still logs sightings. The US air force gave up its secret "Blue Book" project on the subject in 1969.

David Clarke, author of Flying Sorcerers - A History of UFO-logy, said yesterday: "The more info the MoD puts on its website, the less credible the case for a cover-up becomes."