Vulnerable children are being exploited by drug dealers expanding their illegal operations from cities to nearby towns, the National Crime Agency (NCA) has warned.
More than 180 gangs, mostly dealing heroin and crack cocaine, are known to have moved into rural and coastal areas where they believe they can make more money because there is less competition, an NCA report found.
Nearly half (42%) of the places affected were coastal towns which had high levels of unemployment, crime and mental health issues, while more affluent areas with good transport links to major cities were also targeted, it said.
Boys as young as 14 - particularly from London and the South East - were being recruited as runners to help gangs sell drugs in nearby towns, while some girls were starting relationships with dealers which had later turned to violence, according to the report.
Some dealers were even using marketing to expand their illegal businesses, handing out business cards and offering buy-one-get-one free deals on drugs, it added.
The NCA said children who were being used as runners faced "significant physical and mental health issues".
The report said: "It is common for runners to conceal drugs internally; they spend time in unhygienic and unsafe environments and can be subjected to violence and fear. There are additional concerns in relation to young women who may be subject to domestic or sexual violence."
Ian Cruxton, the NCA's director of organised crime, said: "This particular criminal approach puts vulnerable adults and children at risk. They are in an unsafe environment, and exposed to violence and fear.
"I ask all parents, teachers and other professionals who work in this sector to be vigilant. We know that gangs target local children who are unknown to social services and in their eyes are less likely to attract police attention."
Some children who were recruited believed they would be rewarded with large sums of money or higher status within a gang, the report found. One police force reported that a child had received £500 a week as a runner.
Many gangs were using violence - including firearms and knives - to push out local dealers and take over homes to set up bases away from their normal cities, the report said.
The NCA said dealers were selling drugs by setting up "county lines" - a term used to describe the telephone number used by gangs outside their normal area to sell to users on the street.
The practice had earned one gang £3,000 a day, while one force said a county line had been active for 10 years.
Karen Bradley, minister for preventing abuse and exploitation, said: "County lines is an emerging national issue which involves the exploitation of vulnerable young people and adults by violent gang members in order to move and sell drugs across the country.
"This trend has been recognised by the Home Office, National Crime Agency and national policing lead, who are improving the operational response to safeguard the vulnerable and target the most violent by ensuring that the more hidden elements of gang crime and exploitation are visible to the police and local partners."