Rob Knox, who plays student Marcus Belby in Harry Potter And The Half-Blood Prince, due out in November, was killed in a scuffle in the south-eastern suburb of Sidcup early on Saturday. Police said a 21-year-old man has been arrested on suspicion of murder.

Knox became the 14th teenager killed in London this year, and the 10th to die from stab wounds.

He played on the same rugby club as another recent teenage victim, Jimmy Mizen, who was fatally struck with a piece of glass outside a south London bakery earlier this month after refusing to take part in a fight.

In a statement, Rob's parents Colin and Sally said: 'Rob was kind and thoughtful and would always help out others. 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.'

The murder will reignite a debate about youth crime, knives, and whether the country's criminal justice system is working.

Although the homicide rate for England and Wales is more than three times lower than that in the United States, according to 2005 statistics, the parade of young victims on the nightly news has disquieted Londoners.

Figures obtained by The Telegraph newspaper show that out of more than 60,000 people prosecuted for possessing knives since 1997, only nine have been given the maximum jail term.

Mr Boris Johnson was recently elected London's mayor on vows to tackle violent crime.

And the Metropolitan Police recently kicked off an aggressive new programme to search anyone they wish for knives without having to justify their suspicions beforehand.

However, England's Children's Commissioner Al Aynsley-Green has warned that the searches could backfire if they build hostility among young people. Calling the increased police powers 'contentious', he said: 'There is a balance here. On the one hand for young people to feel safer by having the presence of the police - but on the other hand making sure the new powers don't create further antagonism by increased stopping and searching.'

Knox, 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 had come to the Metro Bar in Sidcup, Kent and was seen through the window waving one of the knives around.

A scuffle broke out inside the bar and later spilled on to the pavement. Knox and his brother Jamie were reportedly trying to protect each other when the man stabbed the 18-year-old.

The brothers' friend, Tom Hopkins, 18, who was also stabbed in the head, said: 'Rob was just trying to help out. He was like that. I grabbed the knife, I didn't know at the time that he had another knife. It was just chaos.'

Knox was taken to hospital, where he was pronounced dead.

AGENCE FRANCE-PRESSE, ASSOCIATED PRESS