Britain: Half of gangs are based in the African-Caribbean community
This is London:
An influx of immigrants from violent countries is contributing to gang warfare, police have warned.
Scotland Yard said that asylum seekers and other migrants who had witnessed violence in their home countries were having a "disproportionate negative impact" on other youngsters here.
The report is the first official acknowledgment that refugees from war-torn and often lawless countries are at least partly responsible for an upsurge in gang violence.
Police have identified at least 171 gangs operating in London alone.
Criminologists have long argued that youngsters who were forced to act violently to survive in their homeland need help adjusting to a new life in Britain.
But the authorities have shown a reluctance to tackle the issue for fear of stereotyping or appearing racist. However, they already target British-born youngsters at risk of criminal behaviour from birth.
No figures are kept on the number of murders committed by youngsters from violent countries, such as Somalia or Kosovo. The Scotland Yard report, due to be presented to the Metropolitan Police Authority this week, finally confronts the issue.
It says: "There is an indication from both police intelligence and the voluntary sector addressing gang criminality that there is an increase in young people with significant post-traumatic stress resulting from witnessing and being involved in significant violent situations prior to arrival in the UK.
"These young people appear to have a disproportionate negative impact on their peer groups."
The report, written by a Met assistant commissioner, also reveals that a "significant number" of the gang members arrested for gun violence are illegal immigrants.
The document was compiled for the MPA in the wake of growing fears over gun and knife crime. But it also spotlights a list of shocking crimes committed by refugees who have fled to Britain. No specific incidents are listed, but those recently committed include killings by Roberto Malasi, who grew up in the troubled Democratic Republic of Congo.
He was jailed this year for murdering two defenceless women in unprovoked attacks within the space of 15 days.
One victim was shot in the head as she held her baby niece and the other was stabbed numerous times as she got out of a car.
Somali Yusuf Jama, 19, killed PC Sharon Beshenivsky in a botched raid. His older brother Mustaf is also suspected of involvement.
The brothers had been told they could stay in Britain because their homeland was unsafe. Mustaf made a mockery of the ruling by escaping to Somalia - allegedly disguised in a veil - while being hunted by police.
David Green, director of the Civitas thinktank, said: "We are importing 15, 16, 17 and 18-year-olds brought up in countries with an anarchistic warlord culture in which carrying knives and guns is routine.
"That is no exaggeration. We are asking for trouble if we do not confront this issue, and co-ordinate Government policy properly." He claimed "squeamishness" was preventing the Government or the police tackling the issue.
"If people come from a culture which is anarchistic, they are more likely to be violent, but the police will be frightened to target these people.
"The fear is that it would be called stereotyping. Actually, it is a valid group generalisation."
The Met report's findings also include the revelation there are at least 171 street gangs operating in London.
Although 90 per cent of gang participants are male, there are believed to be three female gangs and women are often used "to mind weapons" for brothers and partners.
The document states that half of gangs are based in the African-Caribbean community.
Violent immigrants fuelling crime