The Ministry of Defence had to blank out "uncomplimentary comments" made by officials about members of the public before publishing its UFO files, a newly released document shows.
Following a deluge of requests under the Freedom of Information (FoI) Act 2000, the MoD agreed to disclose its full archive of reported sightings of unexplained sights in the skies.
But a previously secret memo reveals that the files had to be edited for references to military technology, relations with other countries and insulting remarks about the public written by defence officials and police.
The document, now posted on the MoD's website, also shows that officials feared publishing only part of the information would "fuel accusations of a 'cover-up"'.
Thousands of pages from the department's UFO files have been released through the National Archives since 2008, revealing details of hundreds of reports of strange flying objects and encounters with aliens.
The newly released memo to ministers and defence chiefs, dated September 2007, discusses how to handle making the information public.
It notes: "The majority of the files are of low security classification, but include references to air defence matters, defence technology, relations with foreign powers and occasional uncomplimentary comments by staff or police officers about members of the public, which will need to be withheld in accordance with FoI principles."
The document continues: "The MoD is aware of no clear evidence to prove or disprove the existence of aliens, and consequently the files are considerably less exciting than the 'industry' surrounding the UFO phenomena would like to believe."
Dealing with all the FoI requests for information about UFOs was becoming "increasingly costly and time-consuming" by 2007, with the Directorate of Air Staff (DAS) receiving 199 applications in 2005 alone, the memo shows.
Officials realised they would be forced to release virtually all the MoD files within a few years, and decided it would be better to do this in a "structured" way.
The document pours cold water on conspiracy theories suggesting the Government was heavily involved in investigating aliens.
"Contrary to what many members of the public may believe, MoD has no interest in the subject of extraterrestrial life forms visiting the UK, only in ensuring the integrity and security of UK airspace," it says.
The MoD was tasked with recording sightings of UFOs from the end of the Second World War until it shut down its special investigation unit on December 1 last year.
After the unit was closed, defence staff were told to tell the public to stop making UFO reports in an order sent to all UK military establishments.
"Stations that are contacted by members of the public are advised not to encourage them to report a UFO sighting or to expect an investigation to take place," the newly published order reads.
UFO expert Dr David Clarke, who obtained the memo and the order following FoI applications, was instrumental in getting the full MoD files released.
He said: "What appears to have happened is that they had to make the move because of all the pressure they were getting from people making FoI requests. It must have been costing them a fortune.
"The transfer of the files to the National Archives and the decision to close the desk itself seem to be linked. They were clearly pleased to be shot of it."
The fifth instalment of MoD UFO files was released last week, revealing reports of a large triangular UFO hovering in the skies above former home secretary Michael Howard's home near Folkestone, Kent, in March 1997.
Dr Clarke, a lecturer in journalism at Sheffield Hallam University, said two-thirds of the archive - or about 120 files - still remained to be made public.