A teenage actor who is to appear in the new Harry Potter film was stabbed to death in the early hours of yesterday during a fight at a bar.

Robert Knox, 18, was murdered outside the Metro bar in Sidcup, southeast London, as he tried to protect his brother Jamie, 17, from a knifeman.

The attack, which left three other people with serious stab wounds, took place just a few miles from where Jimmy Mizen, a 16-year-old altar boy, was murdered two weeks ago while trying to shield his older brother.

Yesterday it emerged that Knox and Jimmy had played for the same rugby club in Sidcup.

Last night, Robert’s parents, Sally, a buyer for Marks & Spencer, and Colin, who is thought to work in advertising, paid tribute to their son in a statement.

“Rob was kind and thoughtful and would always help out others; he would always spend his last penny on other people instead of himself,” they said.

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 his family and friends.”

Knox, who overcame bullying as a child to break into acting, had just finished filming a small part for Harry Potter and the Half-Blood Prince, which is due at cinemas in November. He plays a Ravenclaw student called Marcus Belby.

Knox was a former pupil at Beths grammar school in Bexley, whose alumni include Steve Backley, the former Olympic javelin thrower. He is the 14th teenager to be violently killed in London since the start of the year.

Police said the killing was not gang-related and yesterday, as friends laid flowers at the murder scene, close to Sidcup railway station, Lee Bentley, manager of the Metro bar, said the attack appeared to have been triggered by a row over the alleged theft of a mobile phone.

“Nine days ago, a guy came to the bar and caused trouble,” said Bentley. “He accused [Knox’s friend] Dean Saunders of stealing his phone and hit him in the face. We cleaned up Dean and barred the man.”

But the man, who is black and in his twenties, returned on Friday night armed with two knives and tried entering the bar, where Knox was a regular drinker.

What happened next is unclear, but Jade Nicholson, an assistant bar manager, said: “I saw Rob go outside and shout, ‘You pulled a knife on my brother, someone call the police’.”

Tom Hopkins, 18, who was drinking at the bar, said: “Rob had been trying to stop the trouble, it wasn’t his fault.

“All I remember was seeing Rob get stabbed in the chest. I ran over and me and my mate Tarik both tackled the black man. I jumped on top of him and he said, ‘I’ve got a knife, I’ve got a knife’. As I tried to grab the knife I didn’t realise he had another one in his other hand and he cut me in the back of the head.

“I was wrestling with him in the bushes and there were a lot of other people who were helping me out. It felt like it went on for six or seven minutes until the police arrived and then I walked around the corner from the bar. That’s when I saw Rob — it’s too horrible to describe what I saw. It was just red blood.”

Police said they were called to “a large disturbance” at the bar on Station Road at 12.07am, but Knox’s life was already seeping away. A friend, who asked not be named, said: “I had him in my arms. He said, ‘I just want some help’ and I laid him on the floor. When the paramedics came, they put pads on his chest to try to revive him, but it was obvious it was too late.”

Knox was taken to Queen Elizabeth hospital in Woolwich, southeast London, where he was pronounced dead just before 1am. A post-mortem is due to be carried out today.

Three other people were seriously injured in the attack, including Saunders, 21, who was stabbed in the neck and remains in hospital. Police said his injuries were serious but not life-threatening. Two other friends, aged 16 and 19, also received treatment.

A 21-year-old man has been arrested on suspicion of murder.

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