Major dark web drug suppliers have started to voluntarily ban the synthetic opioid fentanyl because it is too dangerous, the National Crime Agency has said.

They are “delisting” the high-strength painkiller, effectively classifying it alongside mass-casualty firearms and explosives as commodities that are considered too high-risk to trade. Fentanyl can be up to 100 times stronger than heroin and can easily cause accidental overdoses, particularly when mixed with heroin.

Vince O’Brien, one of the NCA’s leads on drugs, told the Observer that dark web marketplace operators appeared to have made a commercial decision, because selling a drug that could lead to fatalities was more likely to prompt attention from police.

It is the first known instance of these types of operators moving to effectively ban a drug.

O’Brien said: “If they’ve got people selling very high-risk commodities then it’s going to increase the risk to them. There are marketplaces that will not accept listings for weapons and explosives – those are the ones that will not accept listings for fentanyl. Clearly, law enforcement would prioritise the supply of weapons, explosives and fentanyl over, for example, class C drugs – and that might well be why they do this.

“There are also drug users on the dark web who say on forums that they don’t think it’s right that people are selling fentanyl because it is dangerous and kills a lot of people.”

Fentanyl arrived in the UK around 18 months ago and so far is said to have caused around 160 deaths, with fatalities caused by the opioid rising by nearly 30% last year, according to the Office for National Statistics.

One type of fentanyl, carfentanyl, is thousands of times stronger than heroin and O’Brien confirmed that police had made a number of small seizures of the substance in the UK. In the US, fentanyl has taken a significantly more profound hold on the drugs sector and has replaced heroin in many major US drug markets, precipitating a more deadly phase of the nation’s opioid epidemic. The number of overdose deaths associated with fentanyl and similar drugs has grown to more than 29,000 a year, from 3,000 five years ago. Deaths were up by more than 45% in 2017.

O’Brien said that the NCA is working with US law enforcement agencies to prevent the UK from having a similar fentanyl epidemic, though the number of people dependent on opioids in the UK compared to America means it has a much smaller market.

“We are working closely with international partners in terms of how the threat developed there. It’s an emerging new drug, a threat we’re taken very seriously because of what happened in the US,” said O’Brien. The NCA has had a series of successes against UK fentanyl dealers, who typically source the drug from China and then sell it on the dark web. The first fentanyl case to be sentenced in the UK involved Kyle Enos, 25, from Newport who was jailed for eight years in February. Enos had procured the narcotic from China, selling it worldwide and to customers in 30 UK police areas.

Colin Williams, senior NCA investigating officer on the case, said: “We realised within a number of hours we had to deal with this very quickly.”

Even so, as they tracked down the 160 or so clients who had bought fentanyl from Enos to warn them that the drug was deadly, they learned that four of his customers were already dead. “We can’t say whether they took the drug but they were certainly on his [customer] list,” said Williams.

Enos, himself a fentanyl and heroin user, was aware of the risks and had informed each customer that the substance was liable to kill.

O’Brien added: “Every time we take down a dark web vendor we follow up with customers, and when we have done that, a number are turning up dead – there’s a real cautionary tale there.”

Some of the biggest dark web fentanyl suppliers were closed down last year with the most famous – Alphabay – often described as the largest underground market ever seen - shut following a global police investigation.