A shopkeeper was "smashed" in the face by a teenage boy in a fatal row over buying cigarette papers, a court has heard.

Vijaykumar Patel, 49, died of a "catastrophic" head injury after the unprovoked attack outside the minimarket where he worked in Mill Hill, north London.

The teenager, who cannot be named because of his age, is on trial at the Old Bailey for manslaughter.

He allegedly went into the shop with two friends and tried to buy Rizla cigarette papers, but they were sent away for being under 18.

Prosecutor Louis Mably QC told the jury on Monday that the teenager became aggressive and shouted abuse before hurling himself at Mr Patel outside the shop.

The attack was captured on CCTV and "it is possible to see the moment Mr Patel was hit and which boy delivered the blow", he added.

"This boy suddenly lunged at Mr Patel, flew towards him and smashed him in the upper part of his body with his right forearm," he said.

"When the boy smashed into the lower part of his face it caused Mr Patel, with his hands in his pockets, to fall backwards onto the pavement and smash his head.

"It was that moment, that blow causing him to smash his head, that caused his death."

Mr Patel "had not really been involved" in the incident and "was standing with his hands in his cardigan pockets watching what was going on" when he was struck, Mr Mably added.

Another shop worker then chased the boys away with a billboard sign and a broom.

The boys were seen "laughing, joking and happy about what had taken place" as they fled the scene, Mr Mably told the jury.

Mr Patel was found by police unconscious and bleeding in the street on January 6.

He was rushed to St Mary's Hospital in central London and died the next day from an injury to the back his head.

The defendant, from north-west London, denies the charge of manslaughter.