Innocent Nathan Foster, 18, collapsed in a hail of bullets in a cold-blooded act of revenge, the Old Bailey was told. He had been shot by 17-year-old Junior Glasgow as he sat on a low wall with friends on a hot night in August last year.

Glasgow, of Brixton, south London, was found guilty of murder last month. He was told he would have to serve a minimum of 21 years.

There were cheers and claps from the public gallery as Glasgow was sentenced by Judge Peter Rook. He told Glasgow: "At your young age, you have become part of the gun culture which is a blight on some of our big cities."

Glasgow did not have the "emotional maturity" to deal with being robbed of his prized £800 chain and "reacted wholly out of proportion", said the judge.

He added: "You carried out this terrible revenge on someone you mistakenly thought was responsible. You carried out a cold-blooded, premeditated execution in a public place."

The court was told Glasgow went into the cul-de-sac where they all lived in Marcus Garvey Way, Brixton, on his motor-scooter and started firing with an automatic gun.

Mr Foster, who worked with inner-city children using horses, was hit by six bullets including one through the heart.

The court was told that six hours earlier a chain was taken from Glasgow's neck by another youth.

Mr Foster's older sister Shannel told the judge: "Nathan was killed by a coward. He was my best friend and my favourite brother."