An American jet fighter pilot has spoken of they day he was scrambled to try to shoot down a UFO in British airspace more than 50 years ago.

Lieutenant Milton Torres was based in Britain on May 20, 1957 when he received the order: get up there, arm all weapons and fire on sight.

He climbed into his Sabre jet, took off from a Royal Air Force base in Kent and headed east, spotting on his radar the blip that his superiors had judged to be hostile and probably Russian.

The dot indicated an object about the size of a B52 bomber about 24km away and Torres set a course, rockets at the ready, to catch it.

But the aircraft vanished. The blip on the radar was gone too.

Torres has finally spoken of his top secret mission following the declassification of Ministry of Defence files, cited by The Times newspaper, that relate to reported incidents of unidentified flying objects in British airspace.

Torres, now 77, told The Times that the day after he was scrambled from the RAF base he received a visit from an American in a trench coat 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.

That narrative has now been released by Britain's 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," he told The Times.

"On that night I was ordered to open fire even before I had taken off. That had never happened before.

"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 Torres, who was 24 at the time, nor the pilot of a second Sabre flying behind, actually saw what was making the strong blip on their radars.

Since the incident, Torres 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 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, according to The Times.

But David Clarke, a UFO expert and lecturer in journalism at Sheffield Hallam University, has offered an alternative explanation.

He said 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 told The times that he thought it was linked to clandestine flights over the Soviet Union of the American U2 spy plane.

But he added, "this doesn't explain why Milton Torres was scrambled and ordered to open fire".