A film showing the moment a man with a mental age of nine was stabbed to death in the street left a courtroom reeling in horror.

CCTV film of the stabbing of Dean Mayley in Greenford brought shrieks of horror from the public gallery and gasps from the jury at the Old Bailey.

The victim's mother and sister rushed from court in tears when it was played.

A clip, released by police today, shows the moment Mr Mayley was set upon by a gang of armed muggers.

It came as the youth who stabbed Mr Mayley in February was named for the first time today as Jamal Jones, 17, of Acton.

In the clip, Mr Mayley is shown walking home along Ruislip Road in Greenford when three teenagers surround him and demand he hand over his mobile phone.

When he refuses, 17-year-old Jamal Jones points a knife towards his chest before stabbing him in the heart.

In a full clip of the murder shown to jurors which was deemed too graphic to be released, the knifeman is seen lunging at his victim, fatally stabbing him before ripping the blade out in a dramatic sweeping motion.

Jones admitted he wanted to steal Mr Mayley’s mobile phone and told jurors: “I just meant to slice him, to touch the skin.” But the court heard he “laughed and bragged” about the killing in a prison van on the way to court.

Mr Mayley, 24, suffered from brain disorder microcephaly, which made him “trusting and vulnerable”. He probably did not fully understand what the attackers wanted from him, the Old Bailey was told. Today the jury found Jones guilty of murder as the judge condemned the horror of knife crime and the “disastrous loss of life”.

Three other teenagers, Ryan Beresford, 19 of Acton; Michael Mensah, 18, of Greenford; and Miguel Leiba, 17, of Hanwell, were cleared of murder but convicted of manslaughter and attempted robbery. Jones had pleaded guilty to attempted robbery.

Judge Martyn Zeidman QC will sentence them on October 2. Lifting the court order preventing Jones and Leiba from being identified because of their young age, he said: “It is in the public interest that this information is promulgated — the more people realise the horror of knife crime the better.”

He praised the “immense courage” of Mr Mayley’s mother Donna and his family, who wept at the verdict. “What an utter disaster this is for the family. It is yet another example of an entirely innocent person losing his life because of somebody using a knife,” he said. “There are insufficient words to demonstrate the sadness. I just wish there is some way in which I can turn the clock back. These defendants will be in custody for a very, very long time.”

The court heard how Mr Mayley was confronted as he walked home in Ruislip Road on February 7. Three of the teenagers got out of a waiting car and, while Beresford and Leiba blocked Mr Mayley’s path, Jones stabbed him.

As the attackers made off in the vehicle driven by Mensah, passers-by aided the victim, who staggered into the road before collapsing. He died in hospital less than two hours later.

During the trial, prosecutor Simon Denison QC told jurors about the CCTV film, saying: “This is a chilling piece of footage. Dean is walking along the pavement... Jones confronts Dean, he obviously says something to attract his attention. Dean stops and half-turns.

“Jones goes up to him and in his right hand is a knife. It appears he is holding it so passers-by and those in cars can’t see it, initially behind his back and then down by his side ... Then suddenly with Dean just standing there Jones strikes out at him with the knife and stabs him straight into the heart.”

Jones said Mr Mayley was chosen “because he was by himself, an easy target” and said he intended to sell the phone in Shepherd’s Bush.

Outside the Old Bailey Dean Mayley’s mother and sister wept as they called for new laws to combat knife crime.

Donna and Emma Mayley said they were happy with the guilty verdicts and demanded “the longest possible sentences to keep these people off the streets.”

But they added: “These things will keep happening. Week after week, day after day people will be killed and it’s always innocent people who have done nothing wrong – it has to change. It is unacceptable.

“Our lives will never be the same ever, we can’t move on even though we have to learn to move on because Dean was such a big part of our lives.

“I just hope they (the killers) will get the longest possible sentences to keep them off the streets but others will take over, until the law changes.

“They will be coming out. They have a life. They need to think when they go out with a knife, when they put it in their pocket and leave their home they need to be aware.”

Referring to Michael Mensah who had previous convictions for possessing weapons, they added: “When they are out with a knife, a sword and a taser they are not going to have fun with thee things, they are dangerous weapons. ”