A teenage “time bomb” has been detained for four years for killing a shopkeeper who refused to sell him Rizla papers.

Vijaykumar Patel, 49, sustained a catastrophic head injury in an unprovoked attack by the 16-year-old boy outside a minimarket in Mill Hill, north London, in January.

The boy, from Brent, had claimed he was acting in self defence but was found guilty of manslaughter in July after an Old Bailey trial. He cannot be named for legal reasons.

The court heard the boy, who has ADHD and low intelligence, had previous convictions for weapons and for kicking and punching a teacher at his school.

At the time of the killing, he had been on bail and was in breach of a curfew.

Philippa McAtasney QC argued in mitigation that the boy was “not beyond hope and help”.

She said he had been affected by the stabbing of a youth worker friend who had confronted a drug dealer. The boy, had used his “best handwriting” to express his remorse in a letter to the judge.

However, Mr Justice Stuart-Smith said: “He is a time bomb. The record, the facts of this case, the contents of the PSR [pre-sentence report] with the analysis of very high risk of causing significant harm. I do not find the PSR surprising at all.”

He sentenced the boy to an extended sentence of four years’ detention and a further three years on licence.

The court heard the teenager had five alcoholic drinks at a party earlier on the evening of 6 January. He went into the shop with two friends and tried to buy cigarette papers, but they were sent away for being under 18.

The jury was told he became aggressive and shouted abuse before hurling himself at Patel outside the shop. The attack was captured on CCTV and it was possible to see the moment Patel was hit.

Stuart-Smith told the defendant: “The CCTV shows he was simply standing with his hands in his pockets doing nothing.

“What happened next was that you deliberately moved to the left and launched yourself at Mr Patel, taking him off his feet and to the ground. He was completely defenceless and did not move. You struck him.”

Another shop worker chased the boys away with a billboard sign and a broom. The boys were seen laughing and joking as they fled the scene, the court heard.

Patel was found by police unconscious and bleeding in the street. He was rushed to St Mary’s hospital in central London and died the next day from an injury to the back of his head.

The court heard no victim impact statement had been prepared because the Patel family was in India.