Post-mortem examination results revealed that the victim died of a single gunshot to the back of the head with a hand gun.

The 33-year-old was shot at about 2.20pm on Sunday as he rode south on the M40 between junctions 12 and 13, near Leamington Spa. He is thought to have joined the motorway at junction 15.

Preliminary reports suggest that the victim, who was from south London, was travelling with two friends, who are thought to be Polish. He is known to have attended the Bulldog Bash biker festival at Long Marston airfield in Warwickshire, just 15 miles from the crime scene.

The festival, billed as “Europe’s No.1 Biker Party”, finished at 12 noon on Sunday. Run by the notorious Hell’s Angels biker group, it included drag racing, topless bike washes, and music. The headline act was Status Quo. The murder had striking similarities to an incident six years ago when three motorcyclists leaving the Bulldog Bash were shot at from a car on the M40. Police said that shooting was “indiscriminate” and there were no casualties.

Detective Superintendent Ken Lawrence called for the occupants of a green Rover 620 seen near the crime scene to come forward. He said there were similarities to the shootings six years ago but “that does-n’t necessarily mean they are linked”. Officers have ruled out a road-rage attack. Det Supt Lawrence said the Bulldog Bash festival had passed off peacefully and police were therefore considering the possibility that whoever killed the biker used the event as a means of tracing him.

“The event gathers huge numbers of people from across Europe, and if someone was seeking revenge, the event would be a good place to find someone”, Det Supt Lawrence said.

The festival’s chief organiser, a Hell’s Angel who calls himself Bilbo, said: “I knew the lad and you could-n’t wish to meet a better person. We are used to deaths as bikers – people die in accidents – but we don’t expect somebody to get shot. This is murder, plain and simple, and we have no idea why”. He refused to speculate on whether a rival gang may have been responsible. A source close to the Hell’s Angels said it was not uncommon for members to carry weapons, but said there was only a “small chance” one of them would have carried out the shooting.

“They’re not the sort of guys who go round starting fights,” he said. “But they’re the kind of people who if you start an argument with them they won’t back down, and a lot of them are dangerous men if you start an argument.”

The motorway was reopened by police last night.