Robert Knox (right) with mother Sally and brother Jamie

Daniel Radcliffe. (AP)

London, May 25: A teenage actor who appears in the next Harry Potter film was stabbed to death trying to protect his younger brother from a knifeman yesterday.

Robert Knox, 18, who acted alongside Daniel Radcliffe in Harry Potter and the Half-Blood Prince, became the 28th teenager killed in Britain this year, and the 10th in London to die from stab wounds.

Relatives said his role as Marcus Belby in the film, due for release in November, was set to launch his career as an actor.

Knox was a member of the same rugby club as Jimmy Mizen, the 16-year-old “gentle giant” fatally stabbed in nearby Lee, south-east London, two weeks ago.

In a statement, Rob’s parents, Sally and Colin Knox, said: “Rob was kind and thoughtful and would always help out others — he would always spend his last penny on other people instead of himself. The life and soul of the party, he was very outgoing, loved sports, and would always strike up a conversation with people.

“He was respectful to others and adored by all his family and friends. He was an asset to the family.”

The murder will reignite a debate about youth crime, knives, and whether the criminal justice system is working. Figures obtained by The Telegraph show that out of over 60,000 people prosecuted for possessing knives since 1997, only nine have been given the maximum jail term.

Knox, a grammar school boy, is understood to have been fatally stabbed after trying to save his 16-year-old brother, Jamie, from a man armed with two knives.

The man began attacking drinkers outside the Metro bar, south-east London. Witnesses said the attacker had earlier been thrown out by bouncers, but returned in the early hours with several friends.

Tarik Ozresberoglu, 17, a trainee steel worker, described how he tried to stem the flow of blood from Knox’s wounds then rugby-tackled the attacker into submission.

He said he was chatting to Rob when the attacker appeared. “He pulled out two wooden kitchen knives at least 6in long from his waistband, and said ‘Who’s going to make my day then?’

“Girls were screaming. Jamie, Rob’s brother, came over and said ‘a boy has pulled a knife on me’. I held Rob back, but he pushed me out of the way and said ‘he’s threatened my little brother’.

“Rob was angry, but he’s never started any trouble in his life. I think he just wanted to protect his younger brother. Rob went up to the bloke, who stabbed him four times.

“Rob stumbled back, lifted up his top and we saw the blood seeping from his wounds.

“I took my jumper off and tried to wrap it round Rob’s wounds. Then as I was trying to do that, I saw the guy was still stabbing people. I told my friend Charlie Grimley, who had also been stabbed, to look after Rob. Then I ran over to the bloke and rugby-tackled him into a bush.”

Struggling to control his emotions, Ozresberoglu added: “I might have felt a hero if Rob was still here, but what I did is never going to bring Rob back. I might have saved more people from being stabbed but Rob still isn’t here.”

He said he thought Mrs Knox was allowed to hold her son in her arms as he lay dying on the pavement. “She was just crying and crying, sobbing ‘why my Rob?’.”

A number of drinkers were hurt, including Dean Saunders, 21. Russell Wood, 21, said he saw Saunders stabbed in the neck three times

Wood said: “This guy started accusing everybody of nicking his mobile phone. He made me turn out my pockets. The bouncers chucked him out. But the next thing we knew this chap was outside with about five of his mates. Everybody just tried to get the knife off him and in the process, it seemed, too many people got stabbed.”

A spokesman for Warner Brothers, the makers of the Harry Potter films, said: “We are deeply shocked by the news and our thoughts and sympathy are with the family.”

A Scotland Yard spokesman said: “A man aged 21 has been arrested on suspicion of murder. He is being held in custody. We do not believe the incident is gang-related.”