Smoke from e-cigarettes damages DNA and can increase the risk of cancer and heart disease, scientists have warned.

The battery-driven devices, which deliver an instant nicotine “hit” without burning tobacco, have been widely promoted as a safer alternative to cigarettes.

But findings from a new study suggest they are far from harmless, and could pose a serious health risk.

In laboratory tests, mice exposed to e-cigarette smoke had higher levels of DNA damage in the heart, lungs and bladder than those breathing normal filtered air.

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An e-cigarette standing next to a “real” cigarette. Both may increase the risk of cancers, according to new research (Anthony Devlin/PA)

DNA repair systems, which protect against cancer, were also impaired in the animals’ cells.

The US team led by Dr Moon-shong Tang, from New York University School of Medicine, warn that “vapers” may be increasing their risk of life-threatening conditions.

Reporting their findings in the journal Proceedings of the National Academy of Sciences, the researchers wrote: “We propose that ECS (e-cigarette smoke) is carcinogenic and that e-cig smokers have a higher risk than non-smokers to develop lung and bladder cancer and heart diseases.”

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An e-cigarette smoker at the Vape Lab in Shoreditch, east London (John Stillwell/PA)

Similar results were seen when cultured human lung and bladder cells were exposed to nicotine and nicotine derivatives.

The cells were more likely to mutate or undergo tumour-triggering changes than non-exposed cells.

While tobacco smoke contains a host of potentially dangerous chemicals, e-cigarette vapour consists only of nicotine and some relatively harmless organic solvents.

Recent studies have shown that e-cigarette smokers have 97% less of a lung carcinogen known as NNAL in their bodies than tobacco smokers. That is similar to the level seen in people on nicotine-replacement therapy.

However, NNAL levels are still significantly higher in e-cigarette smokers than non-smokers, the authors of the new study point out.

They pointed out that e-cigarettes were rapidly gaining popularity, especially among young people who regard them as harmless.

“It is important to note that many of these e-cig smokers (who) have taken up the e-cig smoking habit are not necessarily doing it for the purpose of quitting TS (tobacco smoking), rather, it is because they are assuming that e-cig smoking is safe,” the scientists wrote.

Currently there were 18 million e-cigarette smokers in the US and 16% of high school students used the devices, said the researchers.

Most inhaled nicotine is quickly broken down into a non-toxic chemical called cotinine, which is eventually excreted in urine.

But a small proportion, less than 10%, is believed to be metabolised into nitrosamines and their derivatives, which include NNAL.

These chemicals are capable of inducing tumours in different organs, the scientists pointed out.