E-cigarettes could be "no better" than smoking regular cigarettes and may even cause cancer, scientists warn.
According to new research, the vapour from the electronic devices was shown to damage or even kill human cells during lab tests.
The finding comes as UK public health officials and Prime Minister David Cameron back 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 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.
