The first in a series of UFO files, dating back to the 1970s, have been released on The National Archives' website.
The Ministry of Defence files include descriptions of alleged Unidentified Aerial Phenomenon or "UFO" sightings and subsequent MoD evaluations of the reports.
Among the reports are sightings in West Sussex.
The MoD has said that to date, independent experts have concluded that there are realistic explanations behind alleged UFO reports, such as aircraft lights or natural phenomena.
"Reports are examined by the Ministry of Defence solely to establish whether UK airspace may have been compromised by hostile or unauthorised military activity," a spokeswoman said.
"If required, sighting reports are examined with the assistance of the Department's air defence experts.
"Unless there is evidence of a potential threat, no further work is undertaken to identify the nature of each sighting reported.
"The Ministry of Defence has no other interest or role regarding UFO matters and does not consider questions regarding the existence or otherwise of extraterrestrial life-forms.
"The MoD receives a large number of requests each year for information relating to UFOs, so to meet demand a new system has been introduced to release these files in a structured way.
"The rest of the files are currently under review and will be added to The National Archives over the next three years."
The MoD already releases information on sightings which, and this can be found on the MoD website, through the FOI Disclosure Log.
To visit The National Archives site click here.