A hand-picked team from CO19, the Metropolitan Police's elite firearms unit, will walk the beat in gun crime hotspots where armed gangs have turned entire estates into "no go" zones.
Local politicians and anti-gun campaigners have reacted with anger at the news that the officers will carry Heckler & Koch MP5 submachine guns – capable of firing up to 800 rounds-per-minute – and Glock semi-automatic pistols.
CO19 currently provides armed support in volatile situations like sieges and terrorist attacks, with its officers on constant call in vehicles around London.
But this is the first time that armed officers will be sent on permanent foot patrol anywhere in the country outside Northern Ireland.
"Historically, CO19 was only called out when someone rang up to report a gun crime," said Inspector Derek Carroll, head of the new unit.
"But a lot of streets in London have young people in postcode gangs, aged 14 and upwards, and a lot of communities feel that they are controlling areas of estates.
"We are looking at gangs that have access to firearms and will be robust in dealing with them and disrupting and deterring them."
The team of 18 constables, led by an inspector and two sergeants, will begin their patrols of Brixton, Haringey and Tottenham on Nov 9, following successful trial schemes.
The officers – some on motorbikes – will carrying out weapon "sweeps" of their neighbourhoods in an effort to deter gang members from carrying guns, and are also intended to be a reassuring presence for residents.
"My view is that just because you carry a gun, it should not affect the way you police," Inspector Carroll added. "We chat to people and they love it."
Unlike their counterparts in the United States, British police officers not routinely carry guns, although armed patrols are frequently deployed in the aftermath of shootings and to guard potential terrorist targets.
In October 2000 armed officers on the beat were temporarily introduced in Nottingham after a string of drug-related deaths.
Jennette Arnold, a Labour London Assembly member for northeast London constituency, said that the patrols threatened to tear up the contract between the community and the police.
"No one asked us or the people I represent if this was acceptable and when they do I shall tell them it isn't. It isn't acceptable to throw away the principle of policing by consent," she said.
Gill Marshall-Andrews, chairwoman of the Gun Control Network campaign group, described the routine arming of officers as a "very retrograde step" and warned that it could lead to higher levels of gun crime.
"This is likely to raise the stakes and encourage more criminals, especially young criminals, to arm themselves," she said.
"Gun crime in this country is very low by international standards and that's largely because there aren't many guns about. Arming police officers sends out all the wrong messages."
The Police Federation, which represents rank-and-file officers, has long campaigned against attempts to arm a larger section of the force, but said it had no objection to the new scheme.
Simon Reed, vice-chairman of the national federation, said that although majority of his members did not wish be to armed, forces must be free to respond to particular threats.
"The ethos will always be that the British police are unarmed, but we need officers to be able to use firearms when appropriate," he said. "My feeling is that the current balance is just about right."
The Home Office declined to comment, saying that the operational use of firearms was a matter for local forces.
Officers from CO19, formerly known as SO19, have been involved in a number of high-profile incidents in the capital, including the shooting of Jean Charles de Menezes at Stockwell station in 2005.
Gun-related crime is on the increase in London with 1,736 gun crimes reported in London between April and September this year – up 17 per cent on 2008.
The problem of turf violence between drugs gangs was highlighted earlier this month with a spate of shootings in north London linked to two Turkish gangs, the Tottenham Boys and the Bombacilar.