An alien with a lemon-shaped head tried to kidnap two men, the Ministry of Defence revealed on Monday.
It was one of hundreds of encounters recorded in the 1980s and 1990s, including a UFO seen at RAF Shawbury where Princes William and Harry are now training.
Here the MoD's former head of extra-terrestrial investigations gives his verdict on the X files:
UFOs have been seen by pilots and police and tracked on radar. Their stories, some really sensational, are in the 4,000 pages of new X files released by the MoD yesterday.
“For three years, I was Britain's real-life version of TV's Fox Mulder, researching UFO phenomena.
One fascinating case I investigated lasted for six hours in March 1993 - a UFO that flew over many parts of the UK. It was witnessed by large numbers of people, including police officers and military personnel at Shropshire bases RAF Cosford and RAF Shawbury.
It was described as an "immense triangular shaped craft" moving menacingly towards Shawbury.
It fired a narrow beam of light at the ground which tracked backwards and forwards as if it were looking for something. A witness heard an unpleasant low-frequency humming sound that sent shock waves through his body.
The UFO was the size of an airliner and from a speed of no more than 40mph, it suddenly accelerated away to the horizon many times faster than a military jet.
A briefing on this incident went to the assistant chief of the air staff, one of Britain's most senior military officers. I remember only too well coming into work the morning after the sightings to find phones ringing off the hook.
The MoD's defence intelligence staff assessed radiation levels taken at the landing site as being "significantly higher" than would normally be found in that area.
The files even show that Lord Hill-Norton, a former Chief of the Defence Staff, blasted the MOD's stance on this incident. He said their insistence that this event was of "no defence significance" was nonsense and totally at odds with the facts.
Another interesting case in the files took place in Belgium in 1990 when two F-16 jets were scrambled to intercept a UFO similar to the Shropshire one being tracked on radar. Numerous times the UFO took rapid evasive action and was never intercepted.
I asked the Belgian authorities for an explanation and it was unsettling-"Yes the incident was real, yes there was a craft that had intruded into our air space but no, we don't have the faintest idea what it was".
Many people are suspicious of the MOD releasing this material. Some say that it is "disinformation" designed to mislead people. Having worked on these files I can say that there isn't a spaceship in a hanger "smoking gun".