Karl Bishop, who stabbed to death the Harry Potter actor Rob Knox, was today sentenced at the Old Bailey to life in prison.

Bishop, 22, was told he must serve a minimum term of 20 years. He was given three other life sentences for wounding Knox's friends in the attack.

Bishop had been found guilty yesterday of murdering Knox, 18, who was stabbed five times outside a club in his home town of Sidcup, south-east London, last May. He injured four of Knox's friends, most seriously Dean Saunders, 23, who was left with permanent spinal damage after being stabbed in the neck, and Andrew Dormer, 17, who was stabbed in the chest as he tried to disarm Bishop.

Knox completed filming on Harry Potter And The Half-Blood Prince, in which he played the part of Marcus Belby, a few days before his death.

Bishop lashed out in revenge following an earlier incident. He had a previous conviction for a knife attack on two men and habitually carried knives.

Knox had gone out of the club to protect his younger brother Jamie, 17, after hearing that Bishop had threatened him.

It later emerged that Bishop was under police investigation for a robbery and burglary but had not been arrested. The Metropolitan police has disciplined two officers and changed its procedures as a result.

Bishop did not return to court to hear the other verdicts and impact statements from Knox's parents, Colin and Sally, after he was convicted of murder.

Colin Knox, 55, told GMTV today that his son had "paid the price for his bravery".

He said his son would have been trying to "tackle this guy, bring him down and wait for the police to come".

"Seeing the guy in court all week, his body language was disgusting. When it comes to the crunch and he's found guilty, he disappears into a hole and he can't face the parents, the friends of Robert and those people he attacked.

"It just shows the guy's values, he's got no morals, no values."

Knox's mother, Sally, 51, said Bishop should have been made to go into court.

"Personally, once he's got his sentence and he's gone, I will not waste my time thinking about him," she said.

"I just think maybe somebody like him may have some kind of disturbed mind, which may not be due to the life he's had, it just may be something in him. I don't know."

They said they wanted to do something in their son's memory, such as setting up a youth centre or a sponsorship scheme for actors.