A teenager in Britain is facing life behind bars for stabbing a 15-year-old boy to death for his bicycle.

Joshua Williams, 18, attacked Alan Cartwright with a knife as he cycled with his friends in north London earlier this year.

Williams claimed to have an alibi, but has been convicted at the Old Bailey of murder and conspiracy to rob.

On the evening of 27 February, Alan was riding with five friends along Caledonian Road, Islington, when Williams and two other youths intercepted them, the court heard.

The youths stepped out in front of the cyclists and managed to take two bikes.

Then the third robber stabbed Alan in the chest in what was a "deliberate and gratuitous" attack, prosecutor Jonathan Rees QC told the court.

Alan, known as 'Little Al' to his family, managed to continue riding for a short while along the road before collapsing. He was pronounced dead at the scene.

Williams, who lived off Holloway Road, north London, turned up at a police station with his parents on 4 March to hand himself in, the trial heard.

When asked why they were there, Williams' mother told an officer it was about the incident in Caledonian Road, saying: "Yes, he's involved, he did it."

As he was taken into custody, Williams was heard by another officer to say: "It's the right thing to do, isn't it?".

But during the trial, Williams claimed he only went to the police station to tell them he was not involved in the stabbing. He said was at a youth club on the night of the attack.

Williams' parents and brother backed up his testimony - but the jury noticed the apparently religious family did not all swear on the Bible in the witness box.

The court also heard the defendant was identified as the third robber in a comment about the CCTV, which was released by police and put on YouTube.

The prosecutor also pointed out that the footage showed the killer was left-handed - which Williams is.

He will be sentenced alongside Shaquille Roberts, 18, and a 16-year-old who have already pleaded guilty to the bicycle robbery.

Alan's family wept in court as the verdict was read out.

His mother Michelle Watson - who was eight months pregnant with her third child when Alan was killed - told of the "unbelievable shock" of losing her son.

The family also called for more stop and searches and tougher sentences for carrying knives to deter young people.