London: The CIA has released a secret history of its investigations into UFO sightings, revealing there was more truth in the popular television series The X-Files than is often believed.

The highly critical report describes often bitter debates between investigators who believed "the truth is out there" and their sceptical bosses.

It records tales of bumbling undercover agents whose activities fuelled a widespread belief that the United States Government was covering up what the agency described as "extra-terrestrial visitations by intelligent beings".

The problem was eventually passed to the agency's physics and electronics division where, in true X-Files style, just one analyst investigated UFO phenomena.

But the fifties equivalent of Fox Mulder was constantly undermined by his boss, described by the CIA history as "a non-believer in UFOs", who tried but failed to declare the project "inactive".

While the CIA investigations concluded that all sightings could be explained, the report says "misguided" attempts to keep them secret led to widespread belief of a government cover-up.

The report, written by Gerald Haines, the official CIA historian, was commissioned by the then CIA director, James Woolsey, in 1993 in the wake of renewed claims of a CIA-led cover-up.

For the first time it calls on documents that the agency hid from UFO enthusiasts who obtained thousands of more mundane files under the Freedom of Information Act. The report, completed in 1997, has been released at the request of the British academic journal Intelligence and National Security and is published in its latest issue. US intelligence began investigating UFO sightings in 1947 following a report by a pilot. - The Telegraph, London