The Ministry of Defence will destroy all future UFO reports it receives so it does not have to make them public, a previously secret memo reveals.
Britain's official UFO investigation unit and hotline were closed down at the start of December.
Since then reports of strange sights in the skies sent to the MoD have been kept for 30 days before being thrown out, the newly released policy document shows.
This stance was adopted so defence officials would not have to publish the information in response to freedom of information (FoI) requests or pass it to the National Archives.
The memo, dated November 11, 2009, sets out the MoD's reasons for shutting its UFO unit and ceasing to invite the public to send in details of sightings.
It notes that the number of reports the department received soared last year, taking up extra resources and diverting staff from "more valuable" defence-related activities.
The MoD recorded 634 UFO sightings in 2009, the second highest annual total after 1978, when there were 750, according to UFO expert Dr David Clarke.
This compares with an average of about 150 reports a year over the past decade.
The memo states: "The dedicated UFO hotline answer phone service and e-mail address serve no defence purpose, and merely encourage the generation of correspondence of no defence value.
"Accordingly these facilities should be withdrawn as soon as possible."
The official document covers what defence officials should do when they receive reports of UFOs in the future.
It says: "Reported sightings received from other sources should be answered by a standard letter and... should be retained for 30 days and then destroyed, largely removing any future FoI liability and negating the need to release future files post-November 30 2009."
The memo reveals that MoD chiefs made a point of not discussing their plans to close the UFO unit with other countries because of fears this could be perceived as part of a global cover-up.
It states: "We have deliberately avoided formal approaches to other Governments on this issue.
"Such approaches would become public when the relevant UFO files are released, and would be viewed by 'ufologists' as evidence of international collaboration and conspiracy."
But the document includes as an annex a printout from the US Department of Defence website explaining that the American government stopped collecting reports of UFO sightings in December 1969.
Dr Clarke, a lecturer in journalism at Sheffield Hallam University, obtained the memo through an FoI request.
He said: "This is the final rubber-stamping of the decision - they just want to totally wash their hands of the UFO business altogether.
"It's just been a millstone around their necks ever since the Cold War. They have decided that whatever they do, it reflects badly on them."
The expert said the MoD's new policy on destroying UFO reports would make it much more difficult to uncover the truth about incidents in the future.
"It's like they're desperately trying to avoid having to answer FoI requests on this subject," he said.
"Even if something quite serious happened, perhaps where there was a near-miss with an airline, the MoD will say, 'we may have had a report on it, but we've destroyed it'."
The MoD is releasing its historic UFO files gradually through the National Archives.
Five instalments have been made public so far, amounting to about a third of the total.