A DEADLY synthetic drug which is up to 100 times stronger than heroin could be present in drug supplies across the UK, the National Crime Agency has warned.

The drug, called Fentanyl, and its analogue, carfentanyl, which is 100 times stronger again than street heroin, are believed to have caused several deaths in the Yorkshire, Humber and Cleveland areas.

A National Crime Agency and West Yorkshire Police operation in April targeted a laboratory suspected of producing fentanyl and carfentanyl. This has led to concerns the substances could have been distributed to drug dealers across a much wider area and drug users in other regions are now also at risk.

The agency warned even in the unlikely event users know their drugs contain fentanyl, the chances of overdosing are high.

Only 0.002g (1/50th) within a typical 0.1g heroin deal is potentially fatal, and the tiny amounts make it almost impossible to effect a controlled dose.

Carfentanyl is fatal in doses as small as 0.00002g, which equates to a few grains.

Tony Saggers, head of drugs threat and intelligence at the National Crime Agency, said: "We have taken the unusual step of appealing to people to be vigilant.

"First, because whilst initial toxicology revealed fentanyl analogues in a small number of these deaths, specific re-testing has started to indicate that the influence of fentanyl is greater than first suspected.

"Second, the NCA’s operation with West Yorkshire Police to locate and disrupt an illicit drugs laboratory during the last 72 hours has indicated that it may be a source for the production of fentanyl and other analogues. In particular we now believe UK customers beyond the north east region are likely to have received consignments of these drugs.

"I am particularly concerned that drug dealers within established heroin markets may have purchased fentanyl, carfentanyl, or similar substances from this facility.

"They may not know how dangerous it is, both to them when they handle it, and to their customers.

"If you have invested in fentanyl to mix with heroin or other drugs, please stop immediately and reduce the risk that more people will die.

"The criminal justice implications of supplying fentanyl mixed into other drugs will inevitably be deemed as aggravating and claiming ignorance of the consequences is no defence."

Rosanna O’Connor, director of drugs, alcohol and tobacco at Public Health England said: "PHE has today issued a drugs alert to medical and emergency services, public health and drugs services, following evidence from the recent deaths in Yorkshire and on-going police investigations.

"The overdose deaths were caused by heroin mixed with fentanyl and carfentanyl, which are so strong that small amounts can lead to overdose.

"We are urging heroin users to be extra careful about what they are taking. They need to look out for each other and be alert to any signs of an overdose, such as lack of consciousness, shallow or no breathing, ‘snoring’, and blueing of the lips and fingertips.

"If possible, they should use naloxone if someone overdoses, and immediately call for an ambulance. We strongly advise all dependent drug users to get support from local drug services."