Using e-cigarettes is no safer than smoking tobacco with nicotine, scientists warned after finding the vapour damages DNA and could cause cancer.
Researchers at the University of California created an extract from the "smoke" of e-cigarettes and used it to treat human cells in a laboratory.
The exposed cells developed DNA damage and died far sooner than untreated ones.
Nicotine-free e-cigarettes caused 50 per cent more DNA strand breaks; for those with nicotine, the damage rose three-fold in eight weeks.
Dr Jessica Wang-Rodriguez, professor of pathology at the university in San Diego, said: "Our study strongly suggests that electronic cigarettes are not as safe as their marketing makes them appear. E-cigarettes on the whole have something to do with increased cell death. Based on the evidence to date I believe they are no better than smoking."
Public Health England says "vaping" is far safer but the World Health Organisation remains concerned. The study used normal epithelial cells, which line organs, glands and cavities such as the lungs. Ones exposed to the vapour showed forms of damage.
The team tested two types of each e-cigarette: nicotine ones caused most damage but vapour from nicotine-free versions was still enough to alter cells.
Scientists know of some troubling chemicals in the products such as formaldehyde, a carcinogen.
The research was published in the Journal of Oncology.