A study found that a typical vaping session caused similar damage to the main artery in the heart as that suffered by those who smoked cigarettes.
The findings appear to fly in the face of advice from Public Health England (PHE), which last year said vaping is 95% less harmful than tobacco and called for GPs to be able to prescribe e-cigarettes on the NHS to help people quit smoking.
Researchers said vaping could damage the aorta in a similar way to smoking as they presented their findings at the European Society for Cardiology congress in Rome, the Daily Telegraph reported, and they would not encourage using such devices.
They found that the effects from a 30-minute session of vaping - seen as a typical habit as e-cigarettes deliver nicotine at a slower rate than cigarettes - were similar to those from smoking a cigarette for five minutes.
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And they warned the UK had "rushed into" championing vaping as a method to help give up smoking.
Experiments were carried out on 24 adults with an average age of 30, examining only the immediate effects of e-cigarettes and smoking.
Professor Charalambos Vlachopoulos, from the University of Athens Medical School, told the conference: "We measured aortic stiffness. If the aorta is stiff you multiply your risk of dying, either from heart diseases or from other causes."
He added: "The aorta is like a balloon next to the heart. The more stiff the balloon is, the more difficult for the heart to pump. It's the most powerful biomarker we have for estimating cardiovascular disease."
Prof Vlachopoulos said that while the long-term risks of vaping remained unknown, he would not recommend their use.
He said: "There could be long-term heart dangers. They are far more dangerous than people realise. I wouldn't recommend them now as a method to give up smoking. I think the UK has rushed into adopting this method."
Rosanna O'Connor, from PHE, told the Telegraph: "Vaping carries a fraction of the risk of smoking yet many smokers are still not aware, which could be keeping people smoking rather than switching to a much less harmful alternative."