Around 150,000 nitazene tablets - a super-strength Class A drug with a higher potency than fentanyl - were unearthed at a "sophisticated" factory in Waltham Forest on 24 October, the Metropolitan Police said.

Detectives working alongside UK Border Agency officials also carried out raids in Enfield, leading to 11 members of a suspected criminal gang being arrested and charged.

A "substantial" amount of other Class A and B drugs, together with a firearm, a pill-pressing machine and more than £60,000 in cash was also seized.

Various hard drives storing £8,000 worth of cryptocurrency, as well as large number of mobile phones and laptops, were also taken away.

The raids were carried out between 21 August and 21 November this year as part of a national UK law enforcement effort to investigate the rise in synthetic opioids being infiltrated into the drugs supply network.

The 11 suspects have all been charged with conspiracy to supply Class A and B drugs.

It is believed the drugs were sold via the Dark Web, using encrypted chat applications and social media.

Met Police Detective Superintendent Helen Rance, who is leading the investigation, said: "Synthetic opioids have been detected in batches of heroin found in London and across the UK.

"They substantially raise the risk of incredibly serious harm to the user and are believed to be linked to a number of deaths."

The government announced synthetic opioids would be banned in February this year, as part of a Home Office crackdown on drug deaths.

A total of 11 drugs - including 10 different types of nitazene and brorphine, known as "purple heroin", were made Class A substances.

Anyone caught dealing synthetic opioids now faces an up to life behind bars and an unlimited fine.

What are the signs someone has used synthetic drugs?

According to the Met, symptoms that a person has consumed synthetic drugs can include:

• Small, narrowed pupils
• Drowsiness or dizziness
• Loss of or reduced sense of consciousness
• Breathing difficulties
• Cold and clammy skin
• Nausea and vommitting
• Blue or grey lips and/or fingernails
• Low blood pressure or decreased heart rate

Police urged the public to act immediately if they believe someone has these symptoms - and to describe them to emergency services.

Read more:
Inside a secret fentanyl lab
The million-dollar streets strewn with bodies contorted by fentanyl

Synthetic opioids including fentanyl are the most common substances involved in drug overdose deaths in the US.

Fentanyl - which was among the drugs that killed rapper Coolio - claims the lives of more than 70,000 people in the US every year and is the biggest killer of people aged 18-49.

The drug has wreaked carnage in San Francisco where, on average, one person dies every 10 hours from a fentanyl overdose.

President Joe Biden's drug tsar, Dr Rahul Gupta, warned synthetic opioids pose a "global threat".

"It will not be a surprise" if the UK succumbs to a synthetic drug problem, he told Sky News.

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