The man who stabbed to death teenage "Harry Potter" actor Rob Knox was jailed for life on Thursday and told he must spend at least 20 years behind bars.

Karl Bishop, 22, who smirked as the sentence was handed down, murdered 18-year-old Knox in a frenzied revenge attack outside a bar in Sidcup, southeast of London, in May 2007 as the actor tried to protect his younger brother.

Bishop, who was also found guilty of wounding four of Knox's friends during the violent altercation, was found guilty of murder at the Old Bailey on Wednesday, the Press Association reported.

"You are at present a highly dangerous man," the judge Justice Bean told him.

"There is plainly a very significant risk to the public of serious harm caused by your committing further specified offences of violence.

"When you learned that you had killed Rob your only response was to say 'Yeah, sweet'. Your lack of regret, let alone remorse, was truly chilling."

The court heard Bishop had been involved in a row with Knox at the bar the previous week after which he had said "I'm going to come back next week, and someone's going to die".

When Bishop returned the following week he ended up in another confrontation with Knox and his friends.

He went home, armed himself with two kitchen knives and headed back to the bar. On his way, he came across Knox's 17-year-old brother Jamie and threatened him, prompting the young actor to run out of the bar to help him.

In the ensuing melee Bishop stabbed Knox, who had a small part as Marcus Belby in the forthcoming "Harry Potter and the Half-Blood Prince", five times. One wound penetrated a main artery and was to prove fatal.

Prosecutor Brian Altman said Bishop carried "knives like others carry pens in their pockets" and saw stabbing people as an "occupational hazard".

After the murder verdict was announced, the actor's mother Sally Knox vowed to campaign to stamp out knife crime.

"We will work towards making sure he did not die in vain," she said.

His father Colin added: "We are enormously relieved justice has been done, although there is nothing that will compensate for the shattering sense of loss."