In America, conspiracy theories have proliferated ever since a hot air balloon crashed near Roswell in 1947

British defence experts spent 50 years trying to catch a UFO so they could employ alien technology to build superweapons, a declassified dossier reveals.

The files show that the government feared that China or the Soviet Union had impounded a UFO and were using its secrets to develop super-fast warplanes that would be invisible on radar.

Between 1947 and 1997 the Ministry of Defence ran two UFO desks, one public-facing and one where the real work was carried out in the background by defence intelligence experts.

The Unidentified Aerial Phenomena report, part of three files set to be released, said that the RAF was “particularly interested in any novel technologies which might be useful to their programmes . . . Propulsion, stealth and any novel