Drug producers are capitalising on the rise of vaping to sell a liquid form of “spice” – an illegal synthetic substance nicknamed “zombie” – to unwitting customers.

Suppliers of Kronic Juice, which is being shipped into the UK labelled as a harmless e-liquid, refuse to disclose the ingredients, referring only to a “patent pending flavour enhancer”. However, laboratory tests commissioned by the Observer found it to contain a strain of synthetic cannabis that was banned in 2016.

The same chemical, sprayed on to plant matter and smoked in cigarette papers, was one of the so-called “legal highs” that came to be known as the zombie drug.

The emergence of liquid spice is the latest chapter in a battle between law enforcement and manufacturers, which led to the introduction of a blanket ban under the Psychoactive Substances Act in 2016.

The new law removed spice from high-street shops, but the latest revelations have led to questions over what is being done to police the importation of the drug.

Users have said it is many times more potent than herbal cannabis and have reported suffering chest pains, seizures, memory loss and withdrawal. Some have bought Kronic Juice believing it to contain a legal dose of CBD, a non-psychoactive cannabis extract that is reported to have a wide range of health benefits, including easing symptoms of depression and arthritis.

Todd Renje believed he had bought CBD, which he had vaped for the past three years to self-medicate anxiety, having “decided against pharmaceuticals”. The 37-year-old from the United States said: “It was probably within two or three days that I found myself waking up the next morning and vaping a little before work, then taking it to work with me and vaping all day.

“It got to the point where I’m literally vaping this Kronic Juice every 20 or 30 minutes, and if I don’t I get very nauseous, I start shaking, I get sweaty. It went on for months because I was scared to stop.”

When he quit cold turkey, he said he suffered violent withdrawal symptoms. “I went from Friday, Saturday and Sunday with no sleep,” he said. “Vomiting, shaking, sweating, just feeling absolutely miserable. Then I had a seizure. I couldn’t even tell you what happened, I just woke up at hospital.”

Others, however, were aware it contained a psychoactive ingredient. One, who asked not to be named, said: “I really wanted to like it because it’s cheaper than weed and has plausible deniability.” The 20-year-old said he stopped vaping after it made him feel “confused and anxious”. He added: “It was not what I would call a high.”

In the absence of any information from the suppliers, users have taken to online forums to question the drug’s contents. But now tests reveal that Kronic Juice, bought from the company’s website and shipped to London, contains a class-B drug. Known formally as 5F-CUMYL-PINACA, it is illegal to possess, supply or manufacture in the UK.

The National Crime Agency said that it worked to “disrupt the online supply of drugs to the UK” but added that synthetic cannabis was not a “key priority”.

Shadow home secretary Diane Abbott said it illustrated how “the drugs problem is growing and becoming more varied”. She said that increasing the number of border guards was key to tackling drug importation.

Neil Woods, a former undercover police officer who now campaigns for a regulated drug market with the group Law Enforcement Against Prohibition, said the refusal to reveal its contents was “as despicable as you can get”. “But that’s going to happen with prohibition,” he added.

“The obvious answer is regulation. These people can get away with this because there is no regulation to control them.”

The company’s website, which had been operating since 2015, went offline last week, less than 24 hours after an email was sent detailing this newspaper’s finding. The company did not respond to requests for comment.