Drug treatment experts have raised concerns a drop in the supply of illicit drugs to the UK triggered by the lockdown is leading to an increase in the number of users turning to more dangerous alternatives.

A reduction in global travel, increased border restrictions and a slowdown in movement within the UK has resulted in a drop in the supply of illicit drugs, including heroin and the designer drug spice.

The Guardian understands that border guards have noted a recent drop in seizures as traffic into the country subsides, while senior police sources have noted the dip in supply.

But experts have warned that fewer drugs on the streets is not as positive as it sounds, as users are already turning to high-strength alternatives such benzodiazepines, while there is a significant risk of heroin users substituting with fentanyl, a lethal drug 50 to 100 times stronger than heroin.

The number of drug-related deaths is at a record high, according to the most recent official estimates, driven by deaths linked to drug misuse.

The reduced supply is also leading to an increase in the price of some substances, including cannabis, and decreasing the purity of others as dealers cut them with different chemicals to increase bulk, experts have told the Guardian.

Dr Rachel Britton, the director of pharmacy at the drug, alcohol and mental health charity We Are With You, which runs 80 drug treatment services across England and Scotland, said: “We are receiving reports that the coronavirus is affecting the supply of illicit drugs.

“While less drugs on the street may seem like a good thing, what replaces them is usually more dangerous. We know the purity of many drugs is decreasing as dealers cut them with different substances to increase their bulk. This means people often don’t know what they are taking, increasing the chance of overdose.

“We are also hearing reports that to make up for shortages in drugs such as heroin and spice, a large amount of high-strength benzodiazepines have entered the market, which are a lot more potent than normal diazepam.”

A key concern is that users of heroin may turn to the powerful opioid drug fentanyl as an alternative. There have already been reports of a rise in deaths in the UK caused by the drug prior to the pandemic. The drug is typically up to 50 times stronger than heroin, although one type, carfentanyl, is 5,000 times stronger and used as an elephant tranquilliser.

Niamh Eastwood, the executive director of the drugs charity Release, which is setting up a network of people across the UK to provide information on what is going on in the drugs market in light of the coronavirus, said: “One significant concern is the arrival of fentanyl which would be devastating. This drug has been linked to the overdose crisis in the US and Canada, but so far the UK has largely been spared.

“However, even with the absence of fentanyl, the UK already has record levels of drug deaths. If fentanyl or any of its analogues were to start to appear this would be catastrophic.”

Ian Hamilton, senior lecturer in addiction and mental health at the University of York, said: “The problem with fentanyl is it has wreaked havoc in North America because it’s estimated to be between 50 to 100 times stronger than heroin.

“Pre-Covid we’ve not had anything like the problems North America has had but the consensus is that it’s only a matter of time – and Covid could be the crisis that triggers that.”

In the UK, most heroin comes from Afghanistan and while some fentanyl has been produced and supplied from China in the past, that is no longer the case.

“The added problem with fentanyl is that we’re no longer reliant on China because it can be domestically produced,” Hamilton added.

Lawrence Gibbons, National Crime Agency (NCA) drugs threat lead, said: “During the unprecedented situation the UK is facing, the NCA remains committed to relentlessly pursuing organised crime groups who present the most significant threat to the UK, including those trafficking drugs.

“Our work in the illicit drugs space remains a high priority and we’re aware that criminals may adapt to changing circumstances. We are monitoring intelligence and working with partners to ensure that we, and the whole law enforcement system, are alert to any increase, decline or indeed changes in drug supply chains and can respond swiftly.”

A Home Office spokesperson said: “UK law enforcement including Border Force are working to prevent drug trafficking into the UK and to disrupt drugs supply within the UK. This includes targeting serious organised criminal gangs and those who exploit vulnerable people through corrosive county lines drugs gangs.”

• Those seeking assistance with drug use can talk to a trained adviser at We Are With You via its online chat at the website