A drug is being linked to sixty deaths across the country over the last eight months, the National Crime Agency has revealed.
The NCA, which is investigating the use of fentanyl and its variants, warned the toll could rise as they await further toxicology results.
The powerful painkiller is being mixed with heroin and is believed to be responsible for seven deaths in Barnsley as well as dozens more across the UK.
Dealers have been accused of playing 'Russian roulette' with drug 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 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.