A teenager described by a judge as a "time bomb" was on bail when he killed a shopkeeper who refused to sell him Rizla papers.

The 16-year-old boy "launched" himself at Vijaykumar Patel, 49, causing a "catastrophic" head injury in an unprovoked attack outside the minimarket in Mill Hill, north London.

The youth, from Brent, had claimed self defence but was found guilty of manslaughter following an Old Bailey trial and has been jailed today for four years.

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

And 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, who cannot be named for legal reasons, had used his "best handwriting" to express his remorse in a letter to the judge.