Scientists have discovered a potential cancer-causing substance in certain flavours of US e-cigarettes and chewing tobacco ‘in concerningly high levels,’ a study said on Monday.

The substance called pulegone is contained in products with mint and menthol taste, the team of Sven-Eric Jordt from Duke University in North Carolina wrote in the medical journal JAMA Internal Medicine.

US authorities already banned pulegone as a taste additive in food last year. But last week US regulators said they are preparing to issue a ban on flavoured e-cigarettes after a lung disease left hundreds sick and is suspected of causing several deaths.

Together with colleagues, Jordt examined three US e-cigarette brands and a type of chewing tobacco, which contain the substance. They found the pulegone levels were even higher than traditional menthol cigarettes.

‘The tobacco industry has long known about the dangers of pulegone and has continuously tried to minimize its levels in menthol cigarette flavourings, so the levels are much lower in menthol cigarettes than in electronic cigarettes,’ Jordt said.

But the scientists said it is not clear how the intake of pulegone affects the body if it is ingested via e-cigarette or chewing tobacco rather than smoked.

The risk outside the United States is also uncertain.

‘People are dying from vaping and we're looking at it very closely,’ US President Donald Trump said last week.

A similar trend has not been observed in other parts of the world, experts say.

Advocates for e-cigarettes say they are less dangerous and less addictive that regular cigarettes - which have long been proved to cause cancer - and are a way to help people quit smoking altogether.