Armed police patrols will be deployed to London’s gun crime hot spots in the wake of the fatal shootings of three teenage boys in less than a fortnight, Scotland Yard has said.

The recent spate of attacks has also led Britain’s most senior officer to ask the Home Secretary to reduce the age at which the mandatory five-year sentence for gun possession can be imposed, so that teenagers too could receive the minimum term.

The proposals came after Sir Ian Blair, the Commissioner of the Metropolitan Police, hosted an emergency meeting with senior detectives this morning in the wake of the latest killing, which took place yesterday afternoon when 15-year-old Billy Cox was shot dead in his Clapham home.

Officers today appealed to the public for help in finding Billy's killers. They said that the suspects were with him in his house at around 3.30pm yesterday, before he was shot. At around that time, the youngster’s sister arrived home upon which she heard a loud bang. She found Billy inside with a gunshot wound, but neither her attempts at first aid nor paramedics’ efforts were able to save him.

There were no signs of forced entry at the house - a ground floor maisonette in a block of flats - but the back door was open. It is thought likely that the teenager knew his attackers and the case is not believed to be one of mistaken identity. Detectives are investigating whether the shooting was the result of a row that got out of hand or some kind of feud or local dispute.

Sir Ian met officers from the Specialist Crime Directorate, which includes Operation Trident – tasked with investigating gun crime in the black community – before today’s announcement.

He said a “new trend” was emerging of teenagers being involved in killings and serious violent crime but insisted that his force was “absolutely determined” to stop the situation escalating any further.

The Commissioner also disclosed he had spoken with John Reid about Scotland Yard’s hopes for lowering the age at which the mandatory five-year sentence for carrying a gun can be imposed from 21 to 17. The Home Secretary had looked at the proposal “sympathetically”, he claimed.

Meanwhile, the force also revealed it had arrested a man over the killing of Michael Dosunmu, who was shot by gunmen who broke into his home in Peckham last Tuesday, just days after he celebrated his 15th birthday.

Michael’s murder followed that of 16-year-old James Andre Smartt-Ford, who died after he was shot at least twice during an ice disco at Streatham Ice Arena on the night of February 3.

The plans announced today will see a special task force set up to investigate the three shootings and two other murders which have taken place in South London in the last 10 days. It will see an increase in the number of armed patrols covering the most at-risk areas. The patrols will be intelligence-led, with armed officers stopping suspicious cars to provide high visibility reassurance to worried communities.

Sir Ian said gun crime in London had fallen by 14 per cent in the last year, but admitted this was “no comfort to the families involved in the five murders”. There was no evidence that all of the crimes are linked or that they were tit-for-tat, he said, adding though that there was “some evidential leads that link some of them.”

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