The thug who murdered schoolboy Alan Cartwright for his bike in a seven-second street attack was jailed for a minimum of 21 years today.

Joshua Williams, 18, was told he had carried out “a wholly gratuitous and senseless piece of violence motivated by inquisitive greed.”

Williams stood expressionless with his hands deep in his pockets in the dock at the Old Bailey as Judge Rebecca Poulet QC passed sentence.

But as he was led to the cells cries of “scumbag” rang out from the public gallery.

Alan’s mother Michelle Watson wept bitterly at the back of court although her call for more police powers to cut knife crime today won a raft of support.

Tory Mayor hopeful Zac Goldsmith was among MPs calling for a return of stop and search to deter the routine carrying of knives in public.

Williams was convicted yesterday of murder and conspiracy to rob in the attack in the Caledonian Road in February.

At the time Williams was on police bail after being arrested on suspicion of possessing a four-inch lock knife.

After he was charged with murder the knife case was dropped.

Sentencing Williams to a life term, Judge Poulet said the murder had “rightly horrified and dismayed the public.

“Alan was a wholly innocent boy unconnected to any gang and you have taken life in a pathetic unsuccessful attempt to steal a bike.

“You had readied yourself for the attack with a sharply pointed knife. The whole incident was captured by CCTV.

“We watched you step out with deliberation and swing your left arm at the upper chest. The action is very swift but firm and fatal.

“The knife passed through three layers of clothing and penetrated his body to at least three inches.”

Mrs Watson was eight months pregnant at the time and the judge paid tribute to the way she has had to cope with the “agony, devastation and heartache.”

The judge said the starting point for sentencing a man aged over 18 on these charges was 30 years but she reduced the sentence to reflect Williams’ young age and his intention to cause only grievous harm and not to kill.

After the verdict yesterday the family called for higher sentences for knife crime and a return to stop and search to deter teenagers from carrying knives on the streets.

Home Secretary Theresa May has restricted its use in a bid to improve community relations.

Mr Goldsmith said: “Without a doubt stop and search was being used too much, too randomly, and was alienating whole swathes of the community.

“It was rightly tightened, but there’s a serious concern that things went too far and those carrying knives now no longer fear being stopped by the police.

“Every death is a tragedy, and we are seeing a rapid spike. I think it’s right that we look closely at whether we have got the balance right after all.”

Alan was struck down close to where Ben Kinsella had been knifed to death in 2008 which had sparked a major knife crime campaign.

Witnesses saw Williams laughing as he fled the scene leaving his victim dying in the road.

Alan, a “popular and funny” college student known as “Little Al”, had been cycling along the Caledonian Road with his friends when he was set upon.

Williams, Shaquille Roberts, 17, and a third youth attacked in a “pincer movement” by stepping into the road and trying to knock them off their bikes.

Despite his grievous injury the victim was able to cycle up the road before collapsing outside the Cally swimming pool. He bled to death at the scene.

Yesterday Mrs Watson, 40, said: “I think there should be more stop-and-searches. I think if they stopped people with knives or guns then they can stop them before they go and do something else.”

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.”

Alan’s father, also called Alan Cartwright, 44, said anyone caught with a knife should receive a mandatory sentence of ten years as opposed to four years.

He said: “No one wants to go to prison for stop-and-search, for carrying a knife. It would make people think twice about it.”

Roberts and the other youth will be sentenced at a later date.