The pathologist in the Ben Gardner murder trial told the jury that either the first or third blows caused the brain haemorrhage which killed the IT expert.

IT expert Mr Gardner, 30, had been out celebrating his girlfriend Allana Devine's birthday when he was set upon by a group of men in Throwley Way, Sutton, on November 1 last year.

Dr Ashley Fegan-Earl, who was shown clips of the attack, said Mr Gardner died from a subarachnoid haemorrhage which he described as “bleeding beneath the layer that covers the brain”.

This, Dr Fegan-Earl said, was caused by a “sudden rotation of the head on the neck” tearing a vital artery which supplies blood to the brain - an injury so dangerous it can stop the heart beating instantly.

He said: “There was a sudden gush of blood around the base of the brain which caused cardiac arrest. In natural cases this could be caused by a ruptured blood vessel otherwise it could be from a sudden or unanticipated blow to the head or neck.”

Dr Fegan-Earl confirmed that the initial punch, supposedly thrown by Ross Collender, and the final kick, allegedly from the boot of Daniel Ransom, could have caused the fracture at the base of his skull.

He added that an unanticipated blow to the base of the spine, particularly if the victim was drunk, would cause the head to swivel more violently on the neck.

Mr Gardner, according to a toxicology report, had consumed a moderate amount of alcohol and a small amount of cocaine.

The court heard that subarachnoid haemorrhages are extremely rare and only account for 0.3 per cent of all homicidal deaths.

One eyewitness, Michael Harrison, had earlier described the first punch which caught Mr Gardner off-guard: “It was quite a hard blow, the man didn’t know it was coming. He was then kicked with as much force as you could muster - a sickening kick followed by a dull thud.”

Mr Gardner suffered three separate injuries to his face; one on the left side, where the first punch connected, and two on the right side of the face, possibly from the kick.

The 30-year-old also had a fracture at the base of his skull, which led to the haemorrhage, plus bruising to his chest.

He said: “Any blow which struck the head may have caused the subarachnoid haemorrhage. The first and third blows, seen on CCTV, both hit the head and neck.

“It is also possible that bruising on the chest could be from vigorous resuscitation or even a thump to get the heart going.”