Officers are targeting senior “controllers” who operate branded phone lines to sell their product, and then organise distribution of heroin and crack cocaine to smaller cities and towns.
Children and vulnerable people are frequently used as couriers, and the model has been linked to increases in knife carrying and violence across Britain.
Under Operation Orochi, which has been running since November, 87 county line phone numbers have so far been shut down – around a tenth of the 800 to 1,100 lines estimated to be operating at any time.
A third are believed to originate in London, where police are aiming to take a further 210 numbers out of operation.
There have been 183 charges for drug trafficking and of the 18 cases which have gone to court so far, all defendants have pleaded guilty and received a total of more than 50 years imprisonment.
Metropolitan Police commissioner Cressida Dick said that while previous operations had focused on lower-level dealers who included children and modern slavery victims, police were now targeting senior figures to permanently shut down prolific lines.
“We want to destroy the business model,” she told a remote press briefing on Tuesday. “We are beginning to turn the tables.
“Our operations have become increasingly successful and criminals should be deterred from running county lines because of the ever increasing risk of being caught.”
Dame Cressida said that county lines were not an issue that police could solve alone.
“We need to reduce the demand for drugs and support those addicted to these lethal substances. Local agencies, charities, partners, schools, parents all need to help us protect the most vulnerable in our communities from being exploited by gangs into running drugs across the UK,” she added.
“Lastly, we need to design out the ability to run these lines.”
Dame Cressida said a growing evidence base showed a “glaringly obvious” pattern of phone usage and that the Home Office was in discussions with telecoms companies.
Deputy assistant commissioner Graham McNulty said county lines drug dealing had previously been a “low risk and high reward enterprise” as authorities struggled to understand the new model.
“We are turning that on its head [by targeting controllers],” he added.
“These are the people who often rarely leave London, avoid handling drugs themselves but they coordinate the distribution.
“They exploit children and vulnerable adults, and collect the profits at the end. They remain in the shadows but are responsible for a trail of misery and mayhem across the UK.”
In London in the year to March, 1,300 people linked to county lines were charged with 2,000 offences, including more than 20 murders.
Of those arrested as part of Operation Orochi, 61 per cent have previous convictions for violence and two thirds for weapons carrying.
All but one of the line controllers were male and almost all were using unregistered pay as you go “burner” phones.
Twenty of the lines taken down so far ended in Norfolk, where local officers said they had caused an increase in knife carrying, serious violence, exploitation and public fear.
Det Insp Robin Windsor-Waite, from Norfolk Constabulary, said one group running what was known as the “Tommy line” had around 300 customers buying crack cocaine and heroin in the Norwich area.
Phone records showed that the branded phone number used had sent and received almost 24,000 calls and texts in two months.
The investigation started in November and in January, the line controller was arrested at his home, where officers seized £12,000 of heroin. He was later jailed for four years and two months.
“Traditional wisdom was that we needed to arrest a large number of people to close a line but our experience is that if you get the right person you can close the line immediately and permanently,” Mr Windsor-Waite said.
He said that after closures, users had moved to remaining lines operating in the area and that police were moving from the “big hitters” to the “middle and bottom end of the market”.
During the coronavirus lockdown, an increase in policing capacity has resulted in a nationwide surge in proactive operations against drug dealers.
The Metropolitan Police said that between 16 March and 20 May, drug trafficking arrests had risen by 55 per cent to 2,232 compared to the same period in 2019, and charges up 143 per cent to 841.
Officers said it was too early to know if the pandemic would have a long-term impact on county lines or the demand for drugs, particularly from recreational users.
Mr McNulty said the lockdown had made it more difficult for couriers to go unnoticed on trains and coaches, prompting gangs to make greater use of cars.
“That presents opportunities for us to focus on people in terms of intelligence and evidence,” he added.
“Younger runners are excluded from driving vehicles so we’re perhaps seeing an older cohort engaged at the moment.”
Researchers and charities have raised concerns that children could be more vulnerable to criminal exploitation because of the lockdown and school closures.
But Mr McNulty said that reports of missing children known to be at risk from county lines groups had halved since the end of March, and overall reports were down by 38 per cent.
He warned that it was difficult to understand whether the figures indicated a positive change because of the reduction in contact with teachers and social workers, adding: “We need to wait for them to go back to schools to find out what has happened.”
The Children’s Society said that some line controllers were themselves young people being exploited or adults who were previously groomed.
London service manager Natasha Chopra said: “It’s crucial that children at risk are identified and offered early help, and that those who are already trapped in the cycle of exploitation are recognised as victims and offered support rather than criminalised.
“We want the government to introduce a national strategy to tackle child criminal exploitation, end the current postcode lottery when it comes to supporting young victims.”
Last month, the Home Office announced an additional £5m of funding to enable the Metropolitan Police to continue work against county lines including Operation Orochi.
The National County Lines Co-ordination Centre was set up in 2018 to improve intelligence sharing and action on illicit finance.