Up to 130 drug users may have died in Britain using man-made sedative Fentanyl just since December, fear police.
And detectives fear the sedative 10,000 times stronger than heroin is being mass produced in illegal drug labs all over Britain.
Fentanyl and its event more potent analogue carfentanyl, a drug used to sedate elephants, is being created in kitchen top laboratories to sell to hundreds of drug addicts both in Britain and overseas.
Drug makers are using a supply from Chinese distributors to manufacture the drugs, which is then supplied and exported from Britain.
More than 70 deaths suspected to be related to a spike in fentanyl drug use are awaiting toxicology reports, the National Crime Agency (NCA) has revealed.
The 70 cases are in addition to 60 already confirmed deaths related to fentanyl consumption.
So far police have swooped on two home-laboratories that were exporting fentanyl and its analogue carfentanyl to more than 700 drug addicts, more than 300 of whom live overseas.
Four men, three from Leeds in West Yorkshire and one from Gwent in Wales, have been charged with importing, supplying and exporting fentanyl.
Kyle Enos, 25, is alleged to have used the dark web to purchase fentanyl and other synthetic opioids, which are more than 100 times stronger than morphine.
Carfentanyl is more than 10,000 times stronger than street heroin and 0.00002g is considered to be a lethal dose.
Users are thought to be unaware their drugs are mixed with fentanyl, which can be consumed through touch as well as being inhaled and injected.
Ian Cruxton, deputy director of the NCA, said: “Since December 2016 we have seen a number of drug related deaths linked to fentanyl and carfentanyl.
“The NCA has been working with partners, both in the UK and overseas, to take action against those drug dealers who are playing Russian Roulette with the lives of their customers by mixing synthetic opioids with heroin and other Class A drugs.
“I believe the action taken in the UK by the NCA and our police colleagues has significantly reduced the risk to the public.
“At the same time drug users, their friends and families need to be vigilant.”
Simon Bray, from the national police chief’s council, said: “We see that a substantial part of this particular drug supply operates over the internet and the dark web market places.
“In some cases we see incidences where fentanyl has been mixed with other drugs, in particular heroin.
“We see these drugs posted around the world using a variety of different postal systems.”
He added: “They are fairly small kitchen top affairs.”
Fentanyl and its analogues are Class A synthetic opioids covered under the 1971 Misuse of Drugs Act.