The government took the threat of UFOs so seriously in the 1950s that UK intelligence chiefs met to discuss the issue, newly-released files show.
Ministers even went on to commission weekly reports on UFO sightings from a committee of intelligence experts.
The papers also include a wartime account claiming prime minister Winston Churchill ordered a UFO sighting be kept secret to prevent "mass panic".
"The reason apparently was because Churchill believed it would cause mass panic and it would shatter people's religious views."
Dr Clarke told the BBC: "Since the Freedom of Information Act arrived in 2005, this subject - UFOs - have become the third-most popular subject for people to write to the Ministry of Defence saying 'please could you release this file, or papers that you hold on this particular case'.
"What they've decided to do is to be totally open and to say, 'look we're not holding any secrets back about this subject we've got all these files and we're going to make them available to the public'."
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