Chief constables have been told county line drug operations in worst-hit areas were not being properly targeted by police.

National Crime Agency director-general Lynne Owens made the claim at the National Police Chiefs Council (NPCC) conference on Thursday.

The Times reported that five towns in particular, which were not named, had gangs which were able to run 21 county lines without meaningful law enforcement intervention.

This comes after Norfolk police revealed they had found 21 county lines operating in the county, but the crackdown on drug gangs has seen some successful outcomes.

Norfolk police have said tackling county lines drug activity - a term used to describe a drug network using a dedicated mobile phone line - has been a priority for the force in the last 18 months.

Since the launch of Operation Gravity - a campaign focussed on targeting county lines criminals - more than 760 people have been arrested, 133 of those children under the age of 18.

Figures from the Ministry of Justice also revealed nearly 40pc of all cases at Norwich Crown Court were in relation to drug crime.As the number of cases being dealt with at the court has dropped by almost 25pc in the last four years, drug cases soared by 135pc.

During a week of action against county lines between Monday, October 8 to Sunday, October 14, Norfolk Police arrested 22 people within two days and seized more than 100 wraps of heroin.In just one day, on Friday, October 12, police seized 400 wraps of suspected Class A drugs and arrested five people during a raid in Norwich.

At the conference, Ms Owens emphasised that her comments were not to criticise her colleagues but to emphasise the scale of the threat of organised crime.

Many vulnerable adults on the verge of drug criminality are controlled by gang members through cuckooing, where drug dealers take over their home to use as a base for drug dealing.

Police have been working with partner agencies within housing, education and rehabilitation and local communities to help steer young and vulnerable people away from a life of drug abuse and violence.