Electronic cigarettes raise the risk of lung and bladder cancer and may cause heart disease in just a decade, scientists have warned.
Public Heath England has insisted that using e-cigs, or "vaping", is 95% safer than smoking cigarettes.
But although e-cigarettes do not contain the dozens of cancer-causing toxins in tobacco, they do contain nicotine and other dangerous chemicals.
Now a study by New York University researchers shows human lung and bladder cells exposed to nicotine show clear signs of turning cancerous.
In laboratory tests, mice exposed to e-cigarette smoke had higher levels of DNA damage in the heart, lungs and bladder than those breathing filtered air - and those exposed to 10 years’ worth of nicotine from e-cigarettes also showed signs of their cells turning cancerous.
The US study warns that e-cigarette users, while safer than smokers from lung cancer, remain at a higher risk of contracting the deadly disease than non-smokers. Dr Moon-shong Tang, who led the study, said: "We exposed human cells to the equivalent nicotine (to a) few days’ puffs of an e-cigarette. We found this damaged DNA and suppressed DNA repair, which raises the risk of cancer.
"Scientists in the UK seem to be saying lately that e-cigarettes are harmless, but this evidence shows they are not safe. This is an urgent public health issue."
The research exposed human cells to both nicotine and nitrosamine. While most nicotine is harmlessly passed into the bloodstream or urine, about 10% is believed to be processed into the compound nitrosamine, which evidence suggests causes cancer.
The results show changes in the DNA of human cells and mice which researchers say happen in people before they develop cancer.
About 2.9 million people in Britain use e-cigarettes, despite evidence linking them to bronchitis and increased occurrence of stillbirths.
The World Health Organisation has called on Britain to consider banning the devices from public places because of the dangers of "passive vaping".
Dr Patricia Ragazzon, of the University of Salford, has found similar effects to the US study in lung cells subjected to e-cigarette flavourings.
She said: "While e-cigarettes are healthier than tobacco cigarettes, they do contain toxins and several laboratories have seen evidence of DNA damage and inflammation."
The study, published in the journal Proceedings of the National Academy of Sciences, states: "E-cig smoking is gaining popularity rapidly, particularly in young individuals, and it is important to note that many are assuming that e-cig smoking is safe."
Dr Ed Stephens, of the University of St Andrews, who was not involved in the research, said the study was "consistent with the widely held view that vaping is not without risk of cancer and other diseases, but that risk is usually considerably lower than smoking".
Professor Peter Hajek, of the Tobacco Dependence Research Unit at Queen Mary University of London, said it was not surprising cells were damaged by the tests, but added: "The authors acknowledge the key bit of information that is of crucial relevance in this story - vapers show a reduction in these chemicals of 97% compared with smokers."
Diacetyl, used in e-cigarette flavourings, is linked to a condition called "popcorn lung" in which the lungs become so scarred, a transplant is required. E-cigarette flavourings have been found to contain formaldehyde. Cinnamon and bubblegum flavours may also cause damage to sperm. Metals, including lead and nickel, have been found in e-cigarettes at higher levels than standard cigarettes.