Robert Knox (right) with his mother Sally and younger brother Jamie

An 18-year-old actor in the forthcoming Harry Potter film was stabbed to death during a fight outside a London bar on Saturday.

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.

Police said a 21-year-old man had been arrested on suspicion of murder.

The fatal stabbing takes to 14 the number of teenagers violently killed in the British capital this year.

Campaigners and politicians are worried about the spate of teenage killings.

In a statement, Knox'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."

Film producers Warner Bros said: "We are all shocked and saddened by this news and at this time our sympathies are with his family."

A man came to the bar in Sidcup armed with two knives and was seen waving one of them around through the window.

A scuffle broke out and Knox and his brother Jamie were reportedly trying to protect each other.

Several others sustained knife wounds.

The brothers' friends Tom Hopkins, 18, and Tarik Ozress, 17, managed to pin the armed man down, wrestling with him for several minutes before police arrived.

Hopkins, who was 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 and pronounced dead.

Friends said there had been a row over a phone last week and believe it could have been a revenge attack.

Tags: harry potter, robert knox, stabbing, violence