The order came straight out of the Cold War manual: “Arm all weapons and fire on sight.” For Lieutenant Milton Torres, an American jet fighter pilot based in Britain, it was the first and last time that he had received such a chilling instruction.

As soon as he scrambled his Sabre jet from RAF Manston in Kent and headed eastwards, he saw the blip on his radar, indicating the presence of an aircraft the size of a B52 about 15 miles away, and he prepared to close in for the kill with a salvo of rockets. But the “aircraft”, judged to be hostile and probably Russian, simply vanished. The blip on the radar disappeared.

The 24-year-old American pilot's extraordinary experience on the night of May 20, 1957, which he was officially ordered never to reveal to anyone, has come to light after the declassification of another batch of Ministry of Defence files relating to reported incidents of unidentified flying objects appearing in British airspace — in this case the only known example of a jet fighter pilot being ordered to shoot down a UFO.

Mr Torres, now 77 and a retired professor of civil engineering living in Miami, told The Times that the day after he was scrambled from RAF Manston he received a visit from an American in a trenchcoat who waved a National Security Agency identity card at him and warned him that, if he ever revealed what had happened, he would never fly again.

He took the warning to heart and said nothing until 1988 when, through a solicitor with an interest in ufology, he sent the Ministry of Defence a report giving a full account of the incident. Today his narrative is released by the National Archives.

“I shall never forget it, and for the last 50 years I have been waiting for an explanation, but I've never had one. On that night I was ordered to open fire even before I had taken off. That had never happened before,” Mr Torres said. “I was ready to hit the target with all 24 rockets: it would have been like buckshot out of a shotgun. I asked for authentication of the order to fire and I received it.”

Neither Lieutenant Torres nor his wingman, flying another Sabre behind him, actually saw what was making the strong blip on their radars. In the years since he has become more convinced that the object, travelling at speed and performing manoeuvres beyond the capability of any known aircraft at that time, was an alien UFO.

The airman climbed to 32,000ft and then flattened out, travelling at Mach 0.92, about as fast as the F86D Sabre could go. “The blip was burning a hole in the radar with its incredible intensity. It was similar to a blip I had received from B52s and seemed to be a magnet of light. It had the proportions of a flying aircraft carrier,” he wrote.

The only possible explanation, according to David Clarke, a UFO expert and lecturer in journalism at Sheffield Hallam University, is that in the 1960s it emerged that the CIA had been engaged in a secret project codenamed Palladium, in which advanced equipment was used to create simulated radar blips close to Soviet airspace.

Dr Clarke said that he thought it was linked to clandestine flights over the Soviet Union of the American U2 spy plane. “But this doesn't explain why Milton Torres was scrambled and ordered to open fire,” he said.

Thinking space

The 19 files made available online by the National Archives and covering sightings between 1986 and 1992 include:

— A passenger jet coming in to land at Heathrow nearly colliding with a UFO. The captain of the Alitalia airliner reported seeing a brown missile-shaped object pass overhead near Lydd in Kent in 1991. The MoD ruled out the object being a missile, weather balloon or space rocket and closed an inquiry by the Civil Aviation Authority and the military

— An MoD request that Army and Navy helicopters should not take photographs of crop circles, for fear of undermining the official line that the military did not investigate unexplained phenomena

— A letter from a woman claiming to be from the Sirius system who said her spacecraft — containing two “Spectrans”, pictured below — crashed in Britain during the Second World War

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