A top-secret United States spyplane which flies on the edge of space at five times the speed of sound crashed at the British experimental airbase at Boscombe Down, Hampshire, in September 1994, according to a report in a leading military aviation journal. The SAS, the report said, was scrambled to throw a cordon round the wreckage, which was flown back to the US two days later,

The hypersonic reconnaissance aircraft, called Astra or Aurora, is believed to have been developed in the 1980s as a secret US government "black programme".

Officially, the US denies the aircraft exists, although there have been many reports of mysterious sonic booms and sightings. This is the first report of such an aircraft crashing.

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The British Ministry of Defence and the US Defense Department both denied the story yesterday but David Oliver, the editor of Air Forces Monthly, said that Royal Air Force officers had been among the sources in a two- year investigation of the incident, and that he was sure the report was true.

"We have no doubt that an incident did happen on the day in question and it has never been satisfactorily explained by the authorities", Mr Oliver said yesterday.

Air Forces Monthly is widely read in the MoD and in the defence industry, both in Britain and in the US. The report says the Astra (Advanced Stealth Reconnaissance Aircraft), or AV-6, or Aurora, crashed during take off on runway 23 at Boscombe Down on the evening of 26 September 1994. London Air Traffic Control Centre was alerted that a serious incident had occurred. Later that night a witness reported seeing a tarpaulin screen around the front of the aircraft surrounded by a number of emergency vehicles. The witness said the rear section of the aircraft appeared to be raised, suggesting the nose-wheel had collapsed.

Witnesses also saw and photographed men in plain clothes arriving during the night, who were later identified as SAS. The following day an Agusta 109 helicopter - one of four captured Argentine helicopters used exclusively by the special forces - arrived. "The SAS arrived twice", Mr Oliver said.

The wreckage was kept under a tarpaulin in the corner of a hangar but was seen by witnesses when the doors were opened to bring out a Buccaneer aircraft. The next unusual event was the arrival of a giant C-5 Galaxy transport aircraft which is believed to have flown the wreckage back to the US on 28 September.

Reports of the new aircraft have been around for five years. In April 1992 a US radio ham intercepted a transmission from an aircraft descending from 65,000 feet, a height only reached by the Space Shuttle and the Cold War spyplane the U2 (later renamed TR1). Concorde, the highest-flying and fastest civil aircraft, cruises at 59,000 feet.

In August 1992 unexplained sonic booms were measured over the Netherlands, prompting questions in the Dutch parliament. The path of the aircraft suggested it had flown from RAF Machrihanish on the north-west coast of Scotland. In December that year The Independent reported claims by the crew of a boat in the North Sea that they had seen an Aurora and produced an artist's impression similar to the latest impression in Air Forces Monthly.

On other occasions radio hams intercepted transmissions from unidentified high-flying jets requesting permission to land at Machrihanish, though that airfield has now closed.