A lack of resources and "higher priorities" at the Ministry of Defence prevented a full-scale study of the thousands of UFO reports that have been made since the Second World War, according to official documents.

The observation is contained in a 1995 internal memo from a wing commander in DI55, the branch of the Defence Intelligence Staff (DIS) which was responsible for assessing UFO reports.

The author concludes that there was no longer any reason to continue denying intelligence interest in UFOs, although the public perception of the unit as a "defender of the Earth from the alien menace" was "light years from the truth".

But he thought few people would believe the truth and that lack of funds and higher priorities had prevented a full study of UFO sightings.

Following the memo, a limited investigation of unidentified aerial phenomena (UAP) was commissioned by the MoD.

The secret four-volume report concluded in 2000 that UFOs did not pose a threat, eventually leading to the closure of the UFO desk in 2009.

The UAP report was released to the public following a Freedom of Information request in 2006.

The report's conclusions are summarised as "sightings can be explained as mis-reporting of man-made vehicles, natural but not unusual phenomena and natural but relatively rare and not completely understood phenomena".

The sightings are contained in the eighth batch of files to be released to The National Archives and contains 34 files covering 1985 to 2007.

The files include almost 9,000 pages of UFO sightings and incidents, photographs and drawings, RAF investigations, Freedom of Information requests, parliamentary briefings and government UFO policy documents.