The selection of documents released to the national archives is the ninth tranche of the UK's "X-files" to be made public since the government decided in 2008 that keeping them secret was no longer justified.
Documents from the Ministry of Defence classified archives show staff believed aliens could visit for “military reconnaissance”, “scientific” research or “tourism”.
In a 1995 briefing now published by the National Archives, a desk officer said the purpose of reported alien craft sightings “needs to be established as a matter of priority”, adding there did not appear to be “hostile intent”.
The unnamed official said it was “essential that we start with open minds”, explaining “what is scientific ‘fact’ today may not be true tomorrow”.
Clarifying he did not “talk to little green men every night”, he said: “We have a remit that we have never satisfied. That is, we do not now (sic) if UFOs exist.
“If they do exist, we do not know what they are, their purpose or if they pose a threat to the UK.
“If the sightings are of devices not of the earth then their purpose needs to be established as a matter of priority. There has been no apparent hostile intent and other possibilities are: 1) Military reconnaissance; 2) Scientific; 3) Tourism.”
He added that “if reports are taken at face value” they showed extraterrestrial vehicles had “a very wide range of speeds and are stealthy”.
Thus, he suggested, “we could use this technology, if it exists”.
His briefing document lists possible reasons for UFO sightings, including mass hallucinations, US aircraft, “atmospheric events” and hoaxes, but indicated none provide a fully convincing explanation.
The full archive can be found here: http://ufos.nationalarchives.gov.uk/