The declassification of the final three documents among a series of reports dating from the 1970s brings to an end a 10-year campaign for transparency by ufologists.
The Ministry of Defence (MoD) papers have been trickled out over a series of releases to the National Archives, with many delays along the way.
There are still complaints that they have been made deliberately hard to access, as they are not available online, and require a visit to the National Archives in London to pour through hard copies.
The papers include UFO witness accounts, photos and sketches of mystery objects, and MoD and official reports of sightings.
However, so far there does not appear to be the smoking gun evidence of alien life so-called truth seekers were hoping for.
UFO DISCLOSURE: Nick Pope has welcomed the release of the last batch of British X-Files.
Coming so soon after the revelations about the Pentagon’s UFO program, this will raise suspicion in the UFO community that there’s been co-ordination between the US and UK government on this.
Nick Pope, former UFO investigator at the MoD, has suggested that alien enthusiasts won’t be too chuffed with this final release.
The last three documents all date from 1986, and were released on New Year’s Day.
Mr Pope said: “This looks suspiciously as if The National Archives tried to sneak out this release when nobody was looking.”
The timing of the release, just weeks after the US Pentagon confirmed it had maintained a top-secret UFO investigation program, has also raised suspicions.
Mr Pope added: “Coming so soon after the revelations about the Pentagon’s UFO program, this will raise suspicion in the UFO community that there’s been co-ordination between the US and UK government on this.”
However, he is glad they are finally out.
He added: “Having worked on the MoD’s UFO investigations in the 90s I’m pleased that these files are finally in the open.
“There’s no smoking gun in these files that says UFOs are extraterrestrial, but there are plenty of interesting sighting reports and other related material relating to MoD research on the subject.
“These recent revelations in the UK and the US show that whatever you believe about the UFO mystery, some of us in government took the subject seriously.
“I hope this means that the media, the scientific community and the public can now have a more serious and informed debate about the UFO phenomenon, with less sarcastic comments about little green men.”