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.
The National Crime Agency (NCA), which is investigating the use of the potentially deadly fentanyl and its variants, warned the toll could rise as they await further toxicology results.
The toxic synthetic opioid is being mixed with heroin and in some cases proving fatal, the agency said, as it accused dealers of playing "Russian roulette" with users' lives.
Recent investigations have uncovered that fentanyl and its chemical adaptations are being both supplied in and exported from the UK, the NCA's deputy director Ian 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 said
Fentanyl, which can be legally prescribed as a painkiller sometimes in the form of a patch or nasal spray, is around 50 times more potent than heroin.
A variant known as carfentanyl - used to anaesthetise large animals like elephants - can be up to 10,000 times stronger than street heroin.
The potency means investigating officers often have to wear protective clothing to handle the substance.
The 60 victims, whose post mortem examination results indicated their drug-related deaths were known to be linked to fentanyl or one of its chemical variants, were predominantly men and a range of ages, with none younger than 18, the NCA said.
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.
A raid at a drug-mixing facility in Morley, Leeds, in April 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, who is in custody, is due at Cardiff Crown Court for a hearing on August 29.