More than 4,000 people in London have been recruited by gangs to supply drugs through networks across the UK, new figures show.
Children as young as 11 are being coerced by criminal gangs, according to research by City Hall.
London is the highest exporting area for so-called county lines gangs.
Mayor of London Sadiq Khan said: "We're only scratching the surface of a major national issue that is driving violence in London and across the country."
Criminal networks deliberately target children and vulnerable adults to courier drugs from urban bases out to customers across the country, running phone lines to take orders.
In total, London's Rescue and Response support programme identified 4,013 individuals recruited by gangs during the study period.
Almost half (46%) were aged between 15 and 19, while 29% were aged from 20 to 25. Most were male (89%).
Police chiefs say the gangs affect every force area in England and Wales and are linked to violent crime. It is estimated that about 15% of county lines activity originates in London.
The most targeted counties were Norfolk, Hampshire, Essex, Sussex and Thames Valley.
Criminals offer young people and vulnerable adults money or drugs to lure them into gangs, approaching them in schools, youth clubs, parks and fast-food shops.
During the study period, a total of 568 young people were referred to the Rescue and Response programme - a coalition of London boroughs set up to help under-25s lured into county lines.
Of those referred, 33 have been successfully drawn away from gang activity to date.
The National Crime Agency, which jointly runs the National County Lines Coordination Centre with the National Police Chiefs' Council, has estimated there are more than 2,000 individual deal line numbers in operation in the UK with annual profits for each in excess of £800,000.