Sixty people have died in the UK in the past eight months, in circumstances believed to be linked to a drug more potent than heroin, it has been revealed.
Fentanyl and its analogues - chemical adaptations - are being mixed with heroin and in some cases proving fatal, the National Crime Agency said.
Users are being warned that dealers are playing "Russian roulette" with their lives by lacing heroin and other class A drugs with synthetic opioids.
The NCA and West Yorkshire Police raided a laboratory suspected of producing fentanyl and one of its analogues carfentanyl in April.
Fentanyl is around 50 times more potent that heroin, while carfentanyl can be up to 10,000 times stronger than street heroin.
The 60 victims, whose post mortem results indicated their drug-related deaths were known to be linked to fentanyl or one of its variants, were predominantly men and a range of ages, with none younger than 18, the NCA said.
The agency's deputy director Ian Cruxton said: "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."
While fentanyl can be legally prescribed as a painkiller, sometimes in the form of a patch or nasal spray, carfentanyl is only used as an anaesthetic for large animals including elephants, Mr Cruxton said.
"We believe the illicit supply from Chinese manufacturers and distributors constitutes a prime source for both synthetic opioids and the pre-cursor chemicals used to manufacture them," he added.
Recent NCA investigations have uncovered that fentanyl and its analogues are being both supplied in and exported from the UK, he said.
The April raid at a drug-mixing facility in Morley, Leeds, resulted in three people being charged with conspiracy to supply and export class A drugs.
The NCA said it had identified 443 customers of that "criminal enterprise" - 271 overseas, and 172 within the UK.
A fourth man was charged on Monday night, following a separate investigation in May, after police said they identified him using the so-called dark web to buy fentanyl or synthetic opioids.
Kyle Enos, of Maindee Parade in Gwent, is accused of importing, supplying and exporting class A drugs.
The 25-year-old is due in court in Cardiff at the end of the month.
Following links between fentanyl and deaths this year in the north of England, Public Health England (PHE) said it began an urgent investigation.
Pete Burkinshaw, the organisation's alcohol and drug treatment and recovery lead, said the "sharp increase" in overdoses that had been feared did not appear to have materialised.
He said: "We have been working with drug testing labs and local drug services to get more information on confirmed and suspected cases.
"We do not have a full picture, but the deaths in Yorkshire do appear to have peaked earlier in the year and fallen since our national alert and, encouragingly, our investigations in other parts of the country suggest we are not seeing the feared sharp increase in overdoses.
"Investigations are ongoing and plans are in place for a scaled-up response if necessary."
PHE is working with the Local Government Association to increase the availability of naloxone, an overdose antidote, to drug users and at hostels and outreach centres.