Officials at the Ministry of Defence believed aliens could visit Earth for military research - or simply as tourists.
A massive cache of previously secret UFO documents released on Thursday shows that one desk officer believed it was a "matter of priority" for reports of alien landings to be investigated.
"If the sightings are not of this earth then their purpose needs to be established as a matter of priority," he wrote in the 1995 briefing.
"There has been no apparently hostile intent and other possibilities are: a) military reconnaissance b) scientific c) tourism."
He also speculated that the possible number of UFO sightings could be much higher than previously thought, since "some indications that the reported incidents are only the tip of an iceberg and many people do not wish to risk embarrassment and so do not report sightings".
Another file quotes a UFO Intelligence officer, who says that if alien technology exists it should be used for Britain's own advantage.
"If the reports are taken at face value then devices exist that do not use conventional reaction propulsion systems; they have a very wide range of speeds and are stealthy. I suggest we could use this technology, if it exists," he said.
However, those hoping for a "smoking gun" as to the existence of aliens are likely to be disappointed.
One official in 1979 briefed the House of Lords before a debate, pointing out that the sheer number of planets and stars in the universe meant that it was overwhelmingly unlikely that the Earth would be visited by aliens more than about once per 1000 years.
Another officer writing about the 'daily mechanics' of the UFO desk job at the MoD said that the suggestion of "top secret teams of specialist scientists scurrying around the country in a real life version of the X Files" is "total fiction".
"The closest we have to the post of 'UFO Expert' is probably my own, but this has always been a generalist, nontechnical post with the emphasis on correspondence with the public," the officer said.
"Whilst I have naturally built up some degree of knowledge of various UFO cases as part of my tenure in this post, I often find the best source of information is simply to Google the internet."
This cigar-shaped UFO was photographed by general aviation pilot David Hastings as he piloted a Cessna Skymaster plane over the Mojave Desert on Sept. 9, 1985. There has never been an indication of hoax in this case, which remains unexplained to this day.
An unidentified flying object was photographed by a government employee over the Holloman Air Development Center in New Mexico in 1964. Conspiracy theorists have claimed the photo is proof that the U.S. government has been in contact with aliens.
While descending an Austrian mountain in 1954, photographer Erich Kaiser captured an image of what he called mysterious silvery-white flying objects.
Mexican Air Force pilots filmed strange, brightly lit objects that moved quickly in the skies on March 5, 2004. Some scientists said the phenomenon could have been caused by gases in the atmosphere.
America's most infamous UFO case centers in Roswell, N.M. Some people claimed an alien spacecraft crashed there in 1947; the military said it was a weather balloon.
The Air Force issued "The Roswell Report" in 1997. Countering claims that aliens were recovered at the New Mexico site, the report said military officials picked up 200-pound dummies, above, that were used in an experiment.
Video grab close up. What looks like an alien creature with visible head, neck, arms and body standing in the Brazilian rainforest in Manaus, Brazil. In this Amaz-alien footage, a lone unidentified being appears to stand in the jungle of Brazil -- even arching its back -- just feet away from a bright flashing blue light. In what some allege as proof that Earth is being visited by aliens from another planet, the video was obtained by paranormal writer Mike Cohen who says the video was taken by two British tourists visiting the Manaus region of the Amazon jungle.
A large circle and geometric pattern local residents say were created by a UFO appeared in a rice field in Krasakan hamlet in Sleman, Yogyakarta, on the morning of Jan. 23, 2011.
Stan Romanek claimed that this still image from a three-minute video he shot in 2003 shows an alien looking into his home in Nebraska. In the film, a strange face appears to be popping up and down outside Romanek's window.
This eyewitness drawing of an alleged UFO sighting is in one of 19 once-secret files posted to the British National Archives website. The files cover sightings reported between 1986 and 1992. Although many of the reports were debunked, some remain unexplained.
An investigator in 1989 measures markings that some said were left behind by a UFO in Normandy, France. France began releasing its "X-Files" on UFO sightings in 2007.
President Jimmy Carter, shown here in 1971, reported that he saw a UFO above Leary, Ga., in 1969. He filed a report about the sighting to the International UFO Bureau in 1973.
Astronaut Gordon Cooper, who piloted Mercury and Gemini space missions in the 1960s, once said he saw a "typical saucer shape, double-cylindrical shape, metallic" UFO. He was also outspoken on the idea that some UFOs were interplanetary vehicles visiting Earth.
That note was written by the outgoing officer. The post of UFO desk officer was subsequently listed internally and was described as the most junior of the MoD managerial grades. Four applications were apparently received.
But the files also contain details of several 'sightings'.
Some of the documents relate to a series of aliens seen above West Wales in 1977. One hotel owner said a dome-shaped object landed "like the moon falling down" behind his hotel, from which two "silver suited 'faceless humanoids' emerged and began 'making measurements'".
Another saw a craze for releasing Chinese sky lanterns lead to a spate of reports flooding into the MoD of "golden orbs" seen in the sky.
One woman called the MoD "panicking [and] saying we had to ring her back (She left three messages saying this). She was petrified and so were her friends. She had never seen anything like this before."
Another man from County Durham was more sanguine.
He "saw similar lights but did not think they were UFOs as he didn't believe aliens would want to visit Houghton" reported the MoD.