New research suggests vapour from e-cigarettes could be harmful to cells in the lungs.
Scientists at the University of Birmingham found the vapour produced may damage the cells which are supposed to remove dust particles and bacteria from the organs - leading to some of the effects seen in smokers and people with lung disease.
Researchers acknowledged there are less cancer causing chemicals, but say there may be other risks.
Scientists mimicked vaping in a laboratory, testing the vapour on lung tissues. They say more work is needed, but issued a warning,
"We suggest continued caution against the widely held opinion that e-cigarettes are safe."
But Public Health England published an independent review into e-cigarettes in February, and say they are much less harmful than smoking, encouraging people to use them to help them give up traditional cigarettes.
A spokesperson said, "Our new review reinforces the finding that vaping is a fraction of the risk of smoking, at least 95% less harmful, and of negligible risk to bystanders".
vaping poses only a small fraction of the risks of smoking and switching completely from smoking to vaping conveys substantial health benefits
e-cigarettes could be contributing to at least 20,000 successful new quits per year and possibly many more
e-cigarette use is associated with improved quit success rates over the last year and an accelerated drop in smoking rates across the country
many thousands of smokers incorrectly believe that vaping is as harmful as smoking; around 40% of smokers have not even tried an e-cigarette
the evidence does not support the concern that e-cigarettes are a route into smoking among young people.
The new research was first published in the journal, Thorax.