BILLY Cox may have been a tagged teenage gangster, but his murder has sparked armed patrols, a possible change in the law and statements by the likes of David Cameron that Britain's families are "badly broken".

On Valentine's Day, the 15-year-old boy became the third teenager to be shot dead in south London in less than a fortnight. Outside the ground-floor council flat on the Fenwick Estate in Clapham North, where he bled to death in his sister's arms, one sobbing teenager after another came to lay flowers.

Yet despite the smiling photograph and the touching tributes, it became clear that Billy was no angel. Police said he was tagged and put under 7pm to 7am curfew after receiving a 12-month supervision order for burglary. All around the estate, his "Remer" graffiti tag was sprayed onto the brick walls.

Six weeks ago he left Ernest Bevin school in Wandsworth to attend Fairbridge - a centre for "troubled and challenging" young people in Kennigton. There he attended only two of the eight days of his under16s course.

On top of that, Billy was allegedly a member of the Clap Town Kids, a Clapham gang whose rivals include the PDC Crew in Brixton and the Blood Set in Peckham.

Billy's father Tommy Cox, a local builder, today said his son "was not perfect but he was dearly loved".

"He was waiting for a place at college where he could continue working for GCSEs. We are missing Billy so much.

"I've been watching news about the two other boys shot and thought - these are only kids. They don't deserve that. The way they get hold of guns now is unbelievable." He added.

Family breakdown

Today Conservative leader David Cameron blamed family breakdown for the Billy's "deeply depressing" murder.

"That's what our society's now come to - teenagers shooting other teenagers in their homes at point blank range," he told GMTV.

"This goes beyond any one policy or any one government. I think what we need is to recognise our society is badly broken and we need to make some big changes, starting now.

"When you look at the people caught up in these events, what you see is a complete absence in many cases of fathers, and a complete presence of family breakdown. That, I think, is what's at the heart of it."

Another leading Tory, the shadow trade and industry secretary Alan Duncan, said many youngsters have no fear of authority. Instead, they are living out the shocking story of Lord Of The Flies, in which a group of schoolboys descend into barbarism after being stranded on a desert island.

In a speech to a think tank, Mr Duncan said adults were no longer able to exert discipline over the young, with Britain facing a "collapse of authority".

"The greatest problem we need to address in Britain is that it is steadily becoming decivilised," he added. "We need to empower teachers so they can exert the control that too many parents are unwilling or unable to exert."

His comments come as home secretary John Reid was meeting south London MPs to discuss the spate of shootings. Mr Reid will also meet Metropolitan Police chief Sir Ian Blair and other senior officers to discuss the next steps in combating gun and gang culture across the country.

Sir Ian said he had already spoken to Mr Reid about lowering the age at which the mandatory five-year sentence for carrying a gun could be imposed. Scotland Yard would like to see it lowered from 21 to 17. A Home Office spokeswoman said Mr Reid was "not unsympathetic" to the police commissioner's views.

In a bid to stop the violence escalating, more armed officers will be sent to south London's gun crime hotspots - particularly in Southwark and Lambeth. Officers will use automatic number plate recognition (ANPR) to conduct spot-checks on suspicious cars and mount high-visibility patrols.

'Catch my boy's killers'

Billy's death follows the fatal shootings of James Smartt-Ford, 16 at Streatham ice rink, and Michael Dosunmu, 15, in his Peckham bedroom.

A special taskforce has been set up to help Trident, the police's black-on-black gun crime unit, investigate the murders. They will also look into the fatal stabbing of Javarie Crighton, 21, and a double shooting in Peckham that killed Chamberlain Igwemba, 47, and wounded an unnamed man.

Yesterday Detective Chief Superintendent Helen Ball said Billy was found in his bedroom by his younger sister, Elizabeth, at 3.30pm on Wednesday.

"His younger sister was returning to the address when she heard a 'bang'. She went into the premises and found her brother dying."

A friend helped administer first aid until paramedics arrived, but he died at the scene. Police said earlier he was shot in the chest.

It was too early to say if the shooting was related to gangs or drugs, Det Chief Supt Ball said at the scene. She would not comment on rumours that a group of young men had burst into Billy's home two days before he died and assaulted him.

The officer appealed for anyone who saw people entering the flat in Fenwick Place earlier that day, or leaving it just after 3.30pm. People who may know why Billy was killed, or who know the boy's whereabouts on Valentine's Day, can call 020 8247 4553.

"I want everyone to get behind the police one hundred per cent," Billy's father pleaded. "If you have anything to say, tell the police. Everyone has been talking to reporters down there but they must tell the police first.

"We need to catch my boy's killers."