Young immigrants from violent and war-torn countries are fuelling mayhem and murder on London's streets, according to a new report.
Half of gangs identified by police are African-Caribbean
Research from Scotland Yard says increasing numbers of youths with significant post-traumatic stress are having a negative impact in the city.
There are currently 171 street gangs operating in London says the report.
The findings are due to be presented to the Metropolitan Police Authority later this week.
"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," it states.
"These young people appear to have a disproportionate negative impact on their peer groups."
Some 43% of gangs are estimated to have more than 20 members, while 18% have more than 50 members.
Although 90% of gang participants are male, there are believed to be three female gangs operating in London, and women are often used "to mind weapons" for brothers and partners.
The document states that half of gangs identified by police intelligence are based in the African-Caribbean community.
The report also warns more needs to be done to encourage gang members to leave their violent lifestyle behind.