Harry Potter actor, 18, is stabbed to death in brawl outside bar near London
Rob Knox was a member of the same rugby club as Jimmy Mizen, murdered in a bakery in Lee, South-East London, two weeks ago.
The drama student died around 1am today in an attack in Sidcup, Kent, and was officially pronounced dead at Queen Elizabeth Hospital in Woolwich.
Police arrested a 21-year-old man on suspicion of murder after he was treated in hospital for facial injuries.
Rob's death takes the number of teenagers violently killed in the capital this year to 14. He is the third to be killed this month alone, after Jimmy Mizen, killed on May 10, and Lyle Tulloch, 15, from Peckham, South London, fatally stabbed in a stairway of a block of flats on May 3.
In the new film - Harry Potter And The Half-Blood Prince, due at cinemas in November - Rob, who had finished his work on the set, plays Ravenclaw student Marcus Belby.
Today Rob's cousin Vicky May, 16, confirmed his role in the film and said he had also appeared in an episode of the hit TV police drama series The Bill.
Vicky's father Kevin May, 42, who was Robert's uncle, said: 'Rob was such a decent young man. We're all shattered, his family are in bits, they're devastated.
'It's all such a shock, we can't believe he's died, and in such a pointless way.
'Rob used to play rugby at Sidcup Rugby Club. That's the same club where Jimmy Mizen went, the lad who was murdered near here two weeks ago. I don't know if Rob and Jimmy were friends but they were at the same club.
'Where's it all going to end? When is this violence and the carrying of knives by young people going to stop? Something's got to be done.'
Rob won the part of trainee wizard Marcus Belby and had already appeared in ITV's The Bill
The dispute which led to Rob's murder was today being linked to the alleged theft of a mobile phone involving two of his friends.