A judge criticised Britain's gun culture on Friday as he jailed a teenager for shooting dead an innocent youth worker in a revenge killing.

To cheers from the public gallery at the Old Bailey, Junior Glasgow, 17, was jailed for life for the "cold-blooded, premeditated execution" of Nathan Foster.

The 18 year-old was gunned down in south London last August, as he tried to flee the gunman who had mistakenly blamed him for the theft of his 800-pound gold chain.

Foster, a talented horseman who had just become a father, was shot once in the heart during the Brixton shooting and then another five times as he turned to escape.

Jailing Glasgow - who was conditionally discharged in December 2006 for carrying a large kitchen knife in a public place - for a minimum 21 years, Judge Peter Rook criticised the use of deadly weapons on the streets.

"At your young age, you have become part of the gun culture that blights some of our big cities," he told the defendant.

"You carried out this terrible revenge on someone you mistakenly thought was responsible (for the theft).

"There were many aggravating features in this cold blooded execution in a public place."

He said that judging from the manner the "wholly out of proportion" shooting, "it is clear you intended to kill your victim".

He said that had Glasgow been 18 - he reaches that age next month - he would have been jailed for a minimum 30 years.

During the trial, prosecutor Michael Shorrock told the court the killing had been an act of revenge for an earlier incident when the chain was snatched from Glasgow.

The court heard Glasgow, who denied murder, thought Foster was behind the robbery. It was, in fact, a friend of Foster's who had snatched the chain, prosecutors said.

At the time of the shooting, the court heard, Foster was helping deprived inner city children to ride horses.

After sentencing, Detective Chief Inspector Adnan Qureshi, from the Trident squad which investigates black murders, described the case as tragic.

"Nathan had recently become a father and was giving something back to his community by working part time as a youth worker," he said in a statement outside court.