The Guardian said that documents released under the Freedom of Information Act to two academics showed that ministry officials had hoped to expunge information about the unit, known as DI55, from records routinely released after 30 years.

A defense ministry official, who spoke on condition of anonymity in keeping with departmental policy, said that during the 1970s - at the height of the Cold War - officials were concerned about a Soviet invasion - not extraterrestrial activity.

The ministry "examines any UFO sighting reports it receives thoroughly to establish whether there is any evidence suggest that U.K. airspace has been compromised by unauthorized air activity," the official said.

The latest files were released following Freedom of Information Act requests by David Clarke, a lecturer in journalism at Sheffield Hallam University and his colleague Andy Roberts.

"These documents don't tell us anything about UFOs but they do show how desperate the (ministry) have been to conceal the interest which the intelligence services had in the subject," Clarke said.

Following a request for information on the program in 1976. The ministry's head of security opposed releasing files because they were confidential and of "very little of value to a serious scientific investigator."

"It is undesirable that even a hint of this should become public and we are currently consulting the (Air Historical Branch) on ways of expurgating the official records against the time when they qualify for disclosure," under public records laws, the official said.

Official report on UFO investigations.