Joshua Williams
Joshua Williams stabbed 15-year-old Alan Cartwright to death during an attempt to steal his bicycle. Met Police

A teenager has been sentenced to life in prison for murdering a 15-year-old cyclist during an attempt to steal his bike. Joshua Williams, 18, was one of three teens who attacked Alan Cartwright and his friends as they rode along London's Caledonian Road with friends on 27 February.

Williams was told he would serve a minimum of 21 years at his sentencing at the Old Bailey on Friday 18 September. A day earlier, he had been convicted of murder and conspiracy to rob despite claiming to have been in a youth club at the time of the attack.

Sentencing Williams, Judge Rebecca Poulet QC said: "This was a gratuitous and senseless piece of violence. It was motivated by acquisitive greed and the incident has rightly horrified and dismayed the public." She added that Alan was an innocent boy, telling the defendant: "You have taken his life in a pathetic and unsuccessful attempt to steal a bike."

During the trial, the court heard how Alan was cycling with five friends when they were intercepted by three other teenagers. CCTV footage shows Williams attacking Alan with a knife, while a second teenager lunges at one of his friends and takes his bike. At the same time, a third teen crosses the road from the left-hand side and grabs another of Alan's friends off his bike. Alan managed to cycle on before collapsing a short while later. He was pronounced dead shortly afterwards.

Alan Cartwright
A post-mortem examination gave Alan Cartwright's cause of death as a single stab wound to the chest. Met Police

On 4 March, Williams went to Islington police station with his parents to hand himself in, the prosecution told the court. His mother told an officer he was involved, saying: "Yes, he's involved, he did it."

During his defence, he changed his stance and claimed he had only gone to a police station to deny his involvement. His parents and brother backed him up on court, but prosecutor Jonathan Rees QC told jurors Williams' alibi was in tatters and said his family had "decided to abandon their principles to support him in his lies to the court". It also emerged that at the time of the murder, Williams had been on police bail on suspicion of possessing a 4in (10cm) knife, though he was not charged.

The two other teenagers, Shaquille Roberts, 18, and an unnamed 16-year-old who cannot be identified because of their age, were also due to be sentenced.

Speaking after the trial, Alan's family called for tougher sentences to deter knife crime, and appealed for police to conduct more stop and searches. His sister, Cherrie Ives, 22, said: "Now we have gone through it, we have noticed there is a lot more knife crime. It's all down to stop and searches. There should be more."

His father, Alan Cartwright Snr, 44, and mother Michelle Watson, 40, said: "Alan was just a normal 15-year-old boy - loved his bikes, his family, being out with his friends. He was just a funny little kid, at the end of the day."