E-cigarettes could be "no better" than smoking regular cigarettes and may even cause cancer, scientists have warned.
According to a new study, the vapour from the electronic devices was shown to damage or even kill human cells during lab tests.
The research comes as UK public health officials and Prime Minister David Cameron backed the use of e-cigarettes to help people quit smoking.
An estimated 2.6 million people in the UK use e-cigarettes. They are to be licensed and regulated as an aid to quit smoking from 2016.
But Dr Jessica Wang-Rodriguez, co-author of the latest study, said: "Based on the evidence to date I believe they are no better than smoking regular cigarettes."
The scientists treated cells in Petri dishes with vapour from a nicotine-based e-cigarette and a nicotine-free variety and found that the cells which had been exposed to the vapour were more likely to become damaged or die than those that had not.
Those containing nicotine were also said to be more harmful than those that did not, although the authors said it may not be as a result of the addictive substance.
Dr Wang-Rodriguez, chief of pathology at the San Diego branch of the US Department of Veteran Affairs, added: "There have been many studies showing that nicotine can damage cells. But we found that other variables can do damage as well. It's not that the nicotine is completely innocent in the mix, but it looks like the amount of nicotine that the cells are exposed to by e-cigarettes is not sufficient by itself to cause these changes.
"There must be other components in the e-cigarettes that are doing this damage. So we may be identifying other carcinogenic components that are previously undescribed.
"For now, we were able to at least identify that e-cigarettes on the whole have something to do with increased cell death."
But the results seen in the lab tests would not necessarily be exactly the same in a living person, she said, as the amount of vapour used was "similar to someone smoking for hours on end".
The US researchers, who published their findings in the Oral Oncology journal, concluded: "Our study strongly suggests that electronic cigarettes are not as safe as their marketing makes them appear to the public.
"Vapourised e-cig liquids induce increased DNA strand breaks and cell death.
"Further research is needed to definitively determine the long-term effects of e-cig usage, as well as whether the DNA damage shown in our study as a result of e-cig exposure will lead to mutations that ultimately result in cancer."
This month, Mr Cameron told the Commons he believed e-cigarettes were a "very legitimate" way of improving health following a report from Public Health England which said vaping was 95% less harmful than smoking.
But experts criticised the claim and said the study was based on poor quality evidence, with some links to the tobacco industry.
Public Health England also played down the study's results.
Professor Kevin Fenton, national director of health and wellbeing, said: "While Public Health England will carefully consider new studies and continue to be vigilant, the wider body of evidence consistently finds that e-cigarettes are less harmful than smoking.
"Our recent world-leading review found that e-cigarettes carry a fraction of the risk of smoking - the harmful chemicals found in tobacco smoke, including carcinogens, are either absent in e-cigarette vapour or are at significantly lower levels than tobacco smoke.
"The best thing a smoker can do is quit completely now and forever, and we need to provide smokers with accurate, balanced information on different quitting methods.
"Last year, two out of three smokers who combined e-cigarettes with expert support from a local service quit successfully. Smokers who have struggled to quit in the past could try vaping, and vapers should stop smoking."