Armed police are to patrol areas of south London as part of a special taskforce set up to look into the murder of three teenage boys shot dead there in the past 12 days.
The move, announced by Metropolitan Police Commissioner Sir Ian Blair, follows the fatal shooting of Billy Cox, 15, in his home in Lambeth yesterday.
Armed police will be patrolling gun crime hot spots in Lambeth and neighbouring Peckham in police vehicles as part of the initiative.
Commander Cressida Dick of the Met Police told BBC television that suspicious cars would be checked by armed officers in the two areas as part of "more covert operations".
But Dick added: "You will not see officers with guns routinely walking around south London" and insisted that overall gun crime figures were down.
The temporary taskforce will also investigate two other murders in the area and co-ordinate the Metropolitan Police's response to violent crime involving young people in south London.
It will run alongside Operation Trident, the unit that investigates black-on-black gun crime in London.
Prime Minister Tony Blair said gang culture which centred around drugs and guns "needed to be looked at again".
"It is an appalling situation and even though offences involving firearms overall have been falling in the last year we have got to tackle this issue to do with guns, with gang culture, with the way that it grows among certain groups of young people," Blair told reporters.
The new measures are to be introduced following a special meeting called by Blair and attended by senior officers from Scotland Yard's Specialist Crime Directorate.
The recent spate of shootings has fuelled public concern about shootings in London despite a 14 percent drop in gun crime in the past year.
Police were called to Cox's home on Wednesday afternoon after a relative discovered the teenager with gun shot wounds.
Cox, who was of mixed white and Thai parentage, was pronounced dead at the scene despite efforts by his sister to save him.