Documents released by the National Archives reveal how the issue of unidentified aircraft was discussed by authorities and include this photograph from about March 2004. The doughnut-shaped phenomenon were snapped by a retired RAF officer before sending the photo to his old bosses tasked with investigating unidentified flying objects (UFOs).

This sketch, of a "spaceship" creating crop circles, was part of the information released which also included a reference to UFOs being discussed by the CIA and being debated in the House of Lords in 1979.

The files, which contain more than 8,500 pages of information related to sightings and reports of strange airborne objects, include this sketch of a UFO in Scotland.

RAF investigations, unusual radar detections, parliamentary briefings and documents on the government's UFO policy are contained in the documents.

Details in the archive files show how the workload of the Ministry of Defence's (MoD) UFO desk increased by 50% during 1996-7, a change which was attributed to heightened media interest in the subject around the 50th anniversary of an incident in Roswell, US.

The previously classified records also detail the freedom of information requests and letters from "persistent enquirers" that led to the MoD opening the UFO files for the first time.