The reports, dubbed the “British X-Files”, were due to be released but have been recalled from the National Archives by the Ministry of Defence.
A National Archives spokesman said the files contained “information which is covered by FOI exemption”.
This has fuelled conspiracy theories that the top-secret documents hold sensitive information on the UK’s dealings with alien life.
“This has delayed the release of these files. MoD are now ensuring that all information that is covered by an exemption is identified.”
The National Archives will meet in November to look at information which may have been removed before considering publication.
Nick Pope used to investigate UFOs for the MoD and said the files would contain interesting sightings.
Nick stated that he was unhappy to hear the files had been sent back to the MoD.
He said: “For the MoD to send these UFO files to The National Archives, only for them to be sent back to the MoD, is unprecedented.
“I worked on the MoD”s UFO project, wrote many of the documents that have already been released, and made the official announcement that started the entire release project, and yet even I don”t know what”s going on here.
“Bear in mind that for years the MoD told Parliament, the media and the public that this subject was of very limited interest and of no defence significance, yet suddenly these files are being treated with the same sort of sensitivity that you”d expect to see if they contained Britain”s nuclear secrets.
“All this is taking place in the run-up to the US Presidential election, with WikiLeaks revelations showing that discussions about UFOs have taken place at the highest political level in America.
“This whole situation is like something out of The X-Files.”
It is hoped information from the British X-Files will be published next spring, if there is anything left in them.