The number of “county lines” drugs networks discovered by police has more than doubled in the last three years, exploiting children as young as 11, law enforcement chiefs have warned.

A new report by the National Crime Agency (NCA) warns that there are now more than 2000 county lines in the UK, which allow drugs gangs to transport drugs from major cities into other areas, including Scotland.

Crime bosses use children to transport drugs and cash to regional gangs, the report says.

The majority of county line runners are between 15 and 17 years old, and are often offenders picked up by gangs from a young age.

Each ‘line’ of transport between major drug centres and regional cities can be worth as much as £5000 per day, the NCA said.

Annual national profits from county lines operations are likely to be as high as £500 million, while 10,000 children could be involved.

The NCA cited a drug bust by Police Scotland in Aberdeen that discovered a missing 15-year-old last seen in Liverpool, 76 ‘wraps’ of crack cocaine and 22 wraps of heroin.

An academic study from the University of the West of Scotland revealed last year that children as young as 12 were dealing on the streets of Glasgow.

Most of the heroin and cocaine on Scottish streets comes from the coca plantations of South American and the poppy fields of Afghanistan or Pakistan, through England and a ‘national hub’.

The top hub areas for county lines are London, the East Midlands and Merseyside, Tuesday’s report said.

The name ‘county lines’ refers to the phone lines used by drug dealers to communicate.

Police chiefs told a Home Affairs select committee on Tuesday morning that police forces were better at detecting county lines as a result of a new intelligence-sharing network.

NCA lead for county lines Nikki Holland told the Commons Home Affairs Committee: "[The report] doesn't necessarily indicate a worsening of the problem.

"What it actually indicates is an increasing awareness of law enforcement and our partners as to the scale of the problem."

Speaking later at a media briefing, she said "the intelligence picture is as rich as it's ever been" regarding county lines.

Tuesday’s NCA report also highlighted the practice of ‘cuckooing’, where drug gang bosses take over the house of a vulnerable person for their dealing operation.

The properties’ owners are often drug users themselves, or in debt to criminal gangs.

Gangs often target youngsters who are from poor backgrounds, have been excluded from school, or have previous involvement in crime.

But children from "seemingly stable" backgrounds or without a criminal footprint are also targeted, according to the assessment.

It said offenders carry out recruitment both face-to-face and via social media, offering payments and material possessions victims would be unable to obtain through legal means.

"This is enhanced by offenders' use of social media, on which images and videos of cash, designer clothing, luxury cars and other high value goods are posted, creating a misconception that involvement in crime is rewarding," the report said.