Special Branch was concerned that people hooked on such material could go into a frenzy triggered by the millennium leading to anarchy.

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An undated confidential report to the Metropolitan Police, thought to have been filed around 1998-99, listed concerns about conspiracy theorists who believed the end of the world was nigh.

“Fuel is added to the fire by television dramas and feature films mostly produced in America,” the report said.

“These draw together the various strands of religion, UFOs, conspiracies, and mystic events and put them in an entertaining storyline.”

It added: “Obviously this is not sinister in itself, what is of concern is the devotion certain groups and individuals ascribe to the contents of these programmes.”

^{David Duchovny and Gillian Anderson in the X-Files}

The dossier – called UFO New Religious Movements and the Millennium – was drawn up in response to the 1997 mass suicide by 39 cultists in San Diego known as Heaven’s Gate. The group members were “ardent followers of The X-Files and Star Trek” according to Special Branch.

The secret briefing note was obtained from the Met under the Freedom of Information Act by Sheffield-based British X-Files expert Dr Dave Clarke while researching a new book, How UFOs Conquered the World.

Dr Clarke, who teaches investigative journalism at Sheffield Hallam University, said: “The documents show the police and security services were concerned about the export of some new religious movements concerning UFOs and aliens from the USA in the aftermath of the mass suicide by followers of the Heaven’s Gate.

“It’s no coincidence this occurred around 1997 – which was the 50th anniversary of the birth of UFOs and the Roswell incident – at a time when the net was buzzing with rumours about aliens and cover-ups.”

A Met spokesman said: “We have no knowledge of this.”