Sixty people have died in the UK since December after it is believed they took the dangerous painkiller fentanyl, the National Crime Agency (NCA) has warned.
Most of the those found dead with links to fentanyl were heroin users, the agency said.
Fentanyl, which killed the singer Prince, is around 50 times more potent than heroin and up to 100 times more powerful than morphine.
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However, a chemical adaptation of the synthetic drug called carfentanyl – an elephant tranquilliser – can be up to 10,000 times stronger than street heroin.
The drug – and other adaptations of the opioid – is often mixed with heroin and other class A drugs to bulk out supplies.
Doses can then prove fatal, with dealers accused by the NCA of playing “Russian roulette” with the lives of users.
The agency’s deputy director, Ian Cruxton, has said he believes the synthetic drug is being both supplied in and exported from the UK.
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.”
Earlier this year, a spate of fentanyl-related deaths in the north of England kick started an urgent investigation by police.
However, while the authorities have linked fentanyl – or one of its chemical variants – to all 60 deaths in the UK over the last eight months, the cocktail of drugs in the bodies of users at the post mortem examination stage had made it difficult for the cause of death to clearly be attributed to that one drug alone.
Police have recently carried out a number of raids, with four people arrested in Leeds and Gwent.
They are warning anyone who is aware of a supplier of fentanyl – which often sold on the dark web – to get in touch with the authorities.
Prince’s overdose on the drug in 2016 brought fentanyl to international attention.
Sky News crime correspondent Martin Brunt said: “Police raised the alarm for the first time in April when – after years of seeing evidence of fentanyl being used to cut drugs, particularly heroin, in the US and Canada and the subsequent deaths and overdoses – they found the first evidence of its use in the UK.
He added: “It’s not exactly a new drug – it has been used legitimately for many years in the medical world as a very heavy duty painkiller and in palliative care.”