A new study has shown that vapour from the devices can damage or even destroy human cells, PA has reported.
Researchers now want to conduct further tests to determine the long-term effects of using e-cigarettes, including whether they could cause cancer.
The latest findings contradict information released by Public Health England (PHE), which suggested that e-cigarettes are roughly 95% less harmful than tobacco.
It is estimated that 2.6 million adults in Great Britain use electronic cigarettes.
To test the health implications of using the devices, scientists treated cells with a nicotine-based e-cigarette and a nicotine-free version.
They found that cells which had been exposed to the vapour were more likely to become damaged or die than those that hadn't.
"Based on the evidence to date I believe they [e-cigarettes] are no better than smoking regular cigarettes," said Dr Wang-Rodriguez, chief of pathology at the San Diego branch of the US Department of Veteran Affairs and co-author of the study.
"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."
She added that the results from lab tests aren't necessarily the same as what would be found in a living person. This is because the amount of vapour used was equivalent of a person vaping for "hours on end".
Dr Wang-Rodriguez said: "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."
The research was published in the journal Oral Oncology.
Researchers now hope to conduct further research to determine the long-term effects of using e-cigarettes. They also hope to discover whether DNA damage from vaping could cause cancer.
Speaking to HuffPost UK Lifestyle, Dr Helen Webberley, the dedicated GP for Oxford Online Pharmacy, explained: "This is a good example of early research, known as 'in-vitro' studies, where the experiments are done in test tubes and petri dishes.
"If there are any positive findings at this stage, such as the cell death shown here, then this might prompt researchers to go on to carry out 'in-vivo' experiments on live animals and humans."
She added: "The results from in-vitro studies are interesting, but caution must be taken when extrapolating the results and comparing them with what may happen in the living body.
"Although more work needs to be done to study the short term and long term effects of e-cigarettes, the current thinking is that they are less harmful than traditional cigarettes. Of course, it would be better to smoke nothing at all."
The latest study on vaping contradicts information published earlier this year by Public Health England, which stated there was evidence that e-cigarettes carry a "fraction of the risk of smoking".
Professor Kevin Fenton, director of health and wellbeing at Public Health England, said: "Smoking remains England’s number one killer and the best thing a smoker can do is to quit completely, now and forever.
"E-cigarettes are not completely risk free but when compared to smoking, evidence shows they carry just a fraction of the harm."
He added: "The problem is people increasingly think they are at least as harmful and this may be keeping millions of smokers from quitting."
E-cigarettes are due to be licensed and regulated as an aid to quit smoking from 2016.