A Woolwich thug has been locked up for at least 18 years for stabbing a man to death in an act of street violence sparked by a row over a cigarette in Lee.

Kai Steele, 18, was detained indefinitely at Her Majesty's pleasure after being found guilty of murdering 26-year-old Alex Scott in an alleyway outside the Leegate centre on the evening of August 10 last year.

The Old Bailey was told that Mr Scott was stabbed twice in the chest and collapsed on the pavement some 20 seconds after he threw the first punch.

Steele was just 17 at the time of the fatal attack, which happened after an argument erupted outside an off-licence when a youth asked for a cigarette.

Setting the minimum term he will spend in a young offenders' institution, Judge Timothy Pontius said: "This is yet another example of an utterly needless and unjustified waste of a human life resulting from a senseless decision by a teenage boy to put a knife in his pocket before leaving home.

"As is so often the case, that knife was not taken out with any intention in the defendant's mind to use it to cause anyone serious injury, still less to kill.

"This murder was neither planned nor premeditated but arose, as so often the case in the context of street violence, in a matter of moments.

"That fact makes it all the more tragic that a human life was lost simply because a knife was needlessly carried and so what would have begun and ended with the trading of a few punches instead led to a murder."

Yesterday, prosecutor Simon Denison QC told the court that Steele had a series of past convictions for violent crimes, including muggings, dating back to when he was 12 or 13 years old.

An impact statement from Mr Scott's mother, Rachel Van Dyke, said: "His death was totally unnecessary and I know nothing is ever going to bring Alex back. The only thing I can hope for now is the person responsible gets their just desserts.

"Nothing can ever replace Alex. He was funny, outgoing, he had a good heart and would do anything for anybody."

Detective Chief Inspector Diane Tudway, Homicide and Major Crime Command, said: "Alex Scott's death had its origins in a fairly minor verbal confrontation. Kai Steele stabbed him in an instant with a knife that he decided to take onto the street that day for no reason or justification.

"Alex's death was wholly avoidable and came about through Kai Steele choosing to carry a knife. I hope that the sentence today will act as a deterrent if someone is considering taking a knife or weapon out with them. My thoughts are with Alex's family and friends."

At the opening of the trial, Mr Denison told the court: "Alex Scott was 26 years old when he was killed on the evening of Sunday August 10 last year. His death was as sudden as it was violent and wholly unnecessary.

"Another life taken away in an instant by the use of a knife that was taken on to the street for no reason.

"At about 9.30pm that evening he was outside an off-licence in Eltham Road, south-east London, with a group of friends.

"They were all heavy drinkers and had spent the early evening drinking and gambling at the bookies nearby."

He said "words were needlessly exchanged" between Steele, his friend Dillon Beckford and the victim and his friends which led to a fight in a nearby alleyway.

Mr Denison went on: "Alex Scott played his part by being unnecessarily confrontational and he appears to have thrown the first punch, but there was absolutely no need or justification for what happened next.

"Kai Steele took out a knife that he had in his jacket and he lunged at Alex Scott twice. He stabbed him twice in the left side of his chest.

"Either blow could have killed him. One penetrated the aorta and the other penetrated his heart. Twenty or so seconds after he threw the first punch, Alex Scott collapsed on the pavement, where he died."

Steele's co-defendant Beckford, also 18, was acquitted of murder last week on the direction of Judge Pontius.

Steele, of of Lamport Close, Woolwich, and Mr Beckford, of Lee High Road, had both denied the charge against them.

Throughout the sentencing, Steele, who wore a grey tracksuit, appeared impassive in the dock.