Professor Chris Pudney said the sharp rise in Benzo abuse, a commonly prescribed anxiety drug, could result in a wave of deaths unless urgent action is taken to protect casual users and addicts.

The highest risk, he says, comes from Benzos mixed with synthetic opioids.

Professor Pudney, of the University of Bath, told Sky News: "So the thing that we really started to see is the rise of synthetic opioids being present in benzodiazepines. And the thing that really worries us about those drugs is that you need so little to cause death.

"With synthetic opioids, the challenge is that the class called nitazines have really become prevalent in the UK.

"They're even ten times stronger than fentanyl, and fentanyl is even ten times stronger than heroin. And we can think about the problem the US has with fentanyl deaths being incredibly high.

"We've now actually got something here that's even stronger than that.

"And so the real worry is that not only have you got something that - in amounts that you wouldn't believe, or not even observable to the eye - can kill a human being.

"Now, it's also mixed with another drug that is depressant. You can imagine, that you can very easily just stop breathing.

"And that will be the outcome if someone consumes both of these drugs."

Addiction 'can lead to suicide'

Benzodiazepines like Valium, Xanex and Diazepam are commonly prescribed to treat anxiety. But they have now become one of the most widely used recreational drugs in the UK.

Benzos, as they're more commonly known, can be sold cheaply on the street and are easily available.

Dr Gabriel Shaya, a consultant in Addiction Medicine at the charity Turning Point, said: "Developing benzodiazepine addiction or dependence inherently makes anxiety worse.

"People who go through a phase of withdrawal when they stop taking it, get rebound anxiety symptoms, which only makes mental health worse.

"So it's not uncommon to see increased depression rates or increased rates of depression. And ultimately, that can lead to suicide."

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Since 2012 the number of drug deaths involving benzos has doubled to around 10 deaths per week, according to the Office for National Statistics.

According to UK Addiction Treatment Centres, the number of people seeking help for benzo addiction has also trebled in the last three years.

Some experts say that rise is linked to a crisis in the country's mental health services forcing some people to self-medicate illegally rather than wait weeks, months or years for support.

'It's Russian roulette'

Benzo addict Christopher says he "could do 1,000 tomorrow, that's how easy it is".

He has managed to break his addiction to heroin and crack but he has never escaped the grip of benzos.

"You can take the wrong kind, the mixture could kill you," Christopher says. "Do you know what I mean? It's Russian roulette."

He says the thought of dying from Benzos does not deter him.

"It used to, it used to," he says. "But not now. Nah it doesn't bother me. It doesn't bother me.

"I just don't care. I just like my benzos. If I can get them, I get them."