The Kalashnikov, which fired 26 armour-piercing bullets in three seconds during the 2003 killing of Dave King, a gangster, is the most powerful firearm yet used by hitmenin Britain. Ammunition recovered at the scene originated in the Bulgarian, Yugoslavian and Romanian militaries.

The journey made by the rifle — which passed through the hands of a notorious Belgian gunrunner — was uncovered by Oxfam as part of its campaign against the arms trade.

Three men were jailed this week following the death of King, 32, a former minder to Robbie Williams, in Hoddesdon, Hertfordshire.

King, a 6ft 2in, 17-stone bodybuilder known as Muscles, was killed in a drive-by shooting after leaving a gym. He was not wearing his customary bulletproof vest.

Detectives believe that he may have been targeted as a result of a friendship that had degenerated into suspicion and fear. David Sharma, a former friend of King, is being sought for questioning in connection with the murder.

The dangers posed to the public by career criminals who have acquired war-standard weapons were spelt out by Mr Justice Wilkie at Luton Crown Court.

The murder “was committed in a public street in daylight and involved an automatic firearm, and it is only by great good fortune that no other passers-by were seriously injured or worse”, the judge said.

AK47s have been the weapon of choice for guerrilla movements from Vietnam to Afghanistan, Nicaragua to Palestine, for half a century.

The investigation into the weapon’s origin began when it was discovered in a holdall on the Norfolk Broads the day after the killing.

The National Criminal Intelligence Service found that it was an AK47 derivative, made in Hungary, with a special folding stock (shoulder rest) making it easier to handle at close quarters.

The weapon had been used by the Hungarian prison service but by 1992 was surplus to requirements and was sold to Eastronicom, a company run by Geza Mezosy, a Belgian arms dealer.

Mezosy sold weapons legally and illegally all over the world. His contacts included governments, manufacturers, guerrillas and criminals.

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