Police believe a spate of deaths in the north of England has been caused by heroin laced with fentanyl, an opioid 100 times stronger than street heroin, and its chemical cousin carfentanil, which is 100 times stronger again and used by vets to tranquilise elephants.

And after raiding a backstreet lab suspected of making the substances, they fear carfentanil and fentanyl – whose US street names include “drop dead” and “serial killer” – may now be in drugs around the whole country, not just in Yorkshire, Humberside and Cleveland where the deaths occurred.

Public Health England (PHE) has issued a drugs alert, and in a sign of how concerned police and medical authorities are, Tony Saggers, head of drugs threat and intelligence at the NCA, has issued a statement in which he 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 retesting 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, carfentanil, 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.”

Appealing directly to dealers, Mr Saggers added: “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.”

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Heroin laced with fentanyl and carfentanil has been responsible for an epidemic of deaths among American drug users.

Fentanyl – which can be prescribed for severe pain relief – has also been linked to the death of the pop star Prince, who reportedly took it by accident after swallowing pills from a bottle incorrectly labelled as containing much weaker painkillers.

In the US, some street addicts deliberately use fentanyl and carfentanil for added potency, but many more take them unwittingly because they have been used as a cutting agent in the heroin they buy.

The resulting risks of an accidental overdose can be enormous.

World's 10 deadliest street drugs Show all 10 1 /10 World's 10 deadliest street drugs World's 10 deadliest street drugs Whoonga Whoonga is a combination of antiretroviral drugs, used to treat HIV, and various cutting agents such as detergents and poisons. The drug is widely available in South Africa due to South Africa’s high rate of HIV sufferers, and is believed to be popular due to how cheap it is when compared to prescribed antiretrovirals. The drug is highly addictive and can cause major health issues such as internal bleeding, stomach ulcers and ultimately death Getty World's 10 deadliest street drugs Scopolamine Scopolamine is a derivative from the nightshade plant found in the Northern Indian region of South America (Ecuador, Colombia and Venezuela). It is generally found in a refined powder form, but can also be found as a tea. The drug is more often used by criminals due its high toxicity level (one gram is believed to be able to kill up to 20 people) making it a strong poison. However, it is also believed that the drug is blown into the faces of unexpecting victims, later causing them to lose all sense of self-control and becoming incapable of forming memories during the time they are under the influence of the drug. This tactic has reportedly been used by gangs in Colombia where there have been reports of people using scopolamine as way to convince victims to rob their own homes World's 10 deadliest street drugs Heroin Founded in 1874 by C. R. Alder Wright, heroin is one of the world’s oldest drugs. Originally it was prescribed as a strong painkiller used to treat chronic pain and physical trauma. However in 1971 it was made illegal under the Misuse of Drugs Act. Since then it has become one of the most destructive substances in the world, tearing apart communities and destroying families. The side effects of heroin include inflammation of the gums, cold sweats, a weak immune system, muscular weakness and insomnia. It can also damage blood vessels which can later cause gangrene if left untreated World's 10 deadliest street drugs Crack cocaine Crack cocaine first came about in the 1980’s when cocaine became a widespread commodity within the drug trafficking world. Originally cocaine would have attracted a high price tag due to its rarity and difficulty to produce, but once it became more widespread the price dropped significantly. This resulted in drug dealers forming their cocaine into rock like shapes by using baking soda as a way of distilling the powder down into rock form. People were doing this because it allowed for them to sell cocaine at a lower quantity and to a higher number of people. The side effects of crack cocaine include liver, kidney and lung damage, as well as permanent damage to blood vessels, which can often lead to heart attacks, strokes, and ultimately death World's 10 deadliest street drugs Crystal meth Not just famous because of a certain Walter H White, but also because it is one of the most destructive drugs in the world. First developed in 1887, it became widely used during the Second World War when both sides would give it to their troops to keep them awake. It is also believed that the Japanese gave it to their Kamikaze pilots before their suicide missions. After the war crystal meth was prescribed as a diet aid and remained legal until the 1970s. Since then it has fallen into the hands of Mexican gangs and has become a worldwide phenomenon, spreading throughout Europe and Asia. The effects of crystal meth are devastating. In the short-term users will become sleep depraved and anxious, and in the long-term it will cause their flesh to sink, as well as brain damage and damage of the blood vessels World's 10 deadliest street drugs AH-7921 AH-7921 is a synthetic opioid that was previously available to legally purchase online from vendors until it became a Class A in January 2015. The drug is believed to have 80% of the potency of morphine, and became known as the ‘legal heroin’. While there has only been one death related to AH-7921 in the UK, it is believed to be highly dangerous and capable of causing respiratory arrest and gangrene World's 10 deadliest street drugs Flakka Flakka is a stimulant with a similar chemical make-up to the amphetamine-like drug found in bath salts. While the drug was originally marketed as a legal high alternative to ecstasy, the effects are significantly different. The user will feel an elevated heart rate, enhanced emotions, and, if enough is digested, strong hallucinations. The drug can cause permanent psychological damage due to it affecting the mood regulating neurons that keep the mind’s serotonin and dopamine in check, as well as possibly causing heart failure World's 10 deadliest street drugs Bath salts Bath salts are a synthetic crystalline drug that is prevalent in the US. While they may sound harmless, they certainly aren’t the sort of salts you drop into a warm bath when having a relaxing night in, they are most similar to mephedrone, and have recently been featured throughout social media due to the ‘zombification’ of its. The name comes from the fact that the drug was originally sold online, and widely disguised as bath salts. The side effects include unusual psychiatric behaviour, psychosis, panic attacks and violent behaviour, as well as the possibility of a heart attack and an elevated body temperature World's 10 deadliest street drugs Purple Drank One of the more unusual drugs around at the moment, purple drank was popularised in 90s hip hop culture, with the likes of Jay Z and Big Moe all mentioning it in their songs. It is a concoction of soda water, sweets and cold medicine, and is drunk due to cold medicines high codeine content, which gives the user a woozy feeling. However it can also cause respiratory issues and heart failure World's 10 deadliest street drugs Krokodil Krokodil is Russia’s secret addiction. It is believed that over one million Russians are addicted to the drug. Users of krokodil are attracted to the drug due to its low price; it is sold at £20 a gram while heroin is sold for £60. However, krokodil is considered more dangerous than heroin because it is often homemade, with ingredients including painkillers, iodine, lighter fluid and industrial cleaning agents. This chemical make-up makes the drug highly dangerous and likely to cause gangrene, and eventually rotting of the flesh

An NCA spokesman said: “Even in the unlikely event that users know their drugs contain fentanyl, the chances of overdosing are high. Only 0.002g, amounting to 1/50th of a typical 0.1g heroin deal, is potentially fatal, and the tiny amounts make it almost impossible to effect a controlled dose. Carfentanil is fatal in doses as small as 0.00002g, which equates to a few grains.”

Until the recent spate of deaths, there had been few fatalities linked to fentanyl or carfentanil in the UK.

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But four people died in the Barnsley area of South Yorkshire, on 14 April and two men died the day after in Leeds and Normanton in West Yorkshire. Testing on recovered drugs found traces of fentanyl.

Since these six deaths, there have been similar fatalities recorded by Humberside and Cleveland Police.

The NCA has now said: “Fentanyl and carfentanil are believed to have caused several recent deaths in the Yorkshire, Humber and Cleveland areas.”

And PHE has now issued a drugs alert to medical and emergency services, public health and drugs services.

Rosanna O’Connor, director of drugs, alcohol and tobacco at PHE, said: “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.”

The joint PHE and NCA statement “strongly advised” areas currently identifying or suspecting spikes in heroin-associated drugs deaths to contact local coroners to establish whether fentanyl is routinely screened for in toxicology results.