The release of files about a Ministry of Defence civil servant-turned-UFO expert has been blocked.

Nick Pope has drawn on his experiences working for Britain's official UFO investigation unit for a number of books about aliens and strange sights in the skies.

Fellow UFO researcher David Clarke made a Freedom of Information request in 2007 for internal MoD documents about Mr Pope's decision to speak publicly about his conversion from sceptic to believer in the possibility that extra-terrestrials are visiting Earth. But more than three years later, the Information Commissioner's Office (ICO) has upheld the department's refusal to publish the files because they contain personal information.

Mr Pope worked on the MoD's UFO desk from 1991 to 1994, dealing with possible sightings sent in by the public. He published his first book about the phenomena - Open Skies, Closed Minds - in 1996, although he continued working for the MoD until November 2006.

The MoD's official line is that it has no opinion on whether or not aliens exist but 50 years of UFO reports uncovered no evidence of a potential threat to the UK.

Dr Clarke asked the department to release records of its internal discussions about how to respond when Mr Pope began talking to the media about UFOs in 1995-96.

He obtained an April 1996 memo written by a senior officer in defence intelligence, who said the matter should be approached "very delicately".

But the MoD refused to release other papers about Mr Pope, including some which contain "free and frank advice" from defence officials about how to handle the situation.

The ICO said in its ruling on Dr Clarke's request: "The commissioner feels that this is a finely balanced case and that the complainant has provided well reasoned arguments to support his case. However, in this particular case the commissioner does not believe that release of the information concerned would further the legitimate public interest in any significant way."