The number of people vaping in the UK has reached 3.6 million - about half the number of smokers - figures from Action on Smoking and Health suggest.
The data indicates most vapers are former smokers, with the main reason for using e-cigarettes being to give up tobacco.
The findings come as the US continues to investigate a spate of serious lung injuries linked to vaping.
India, meanwhile, says it will ban e-cigarettes as they pose a health risk.
E-cigarettes allow users to inhale nicotine in vapour rather than breathing in smoke.
The anti-smoking charity Action on Smoking and Health has been monitoring trends in their use since 2012.
Its latest report suggests vaping helped an extra 70,000 people quit smoking in 2017.
It says the number of vapers in the UK has gone from 700,000 in 2012 to 3.6 million in 2019 - and of these:
- 54% have given up smoking
- 40% both smoke and vape
- 6% have never smoked
"It's important that all vapers stop smoking completely, as otherwise they are still exposing themselves to the serious risks of disease and disability caused by smoking," says Prof Ann McNeill, who compiled a review of e-cigarettes for Public Health England.
She adds: "Vaping isn't risk free - but it's much less risky than smoking, which kills nearly 100,000 people a year in the UK."
The figures come amid a backlash against e-cigarettes.
The US Centers for Disease Control is investigating a mysterious outbreak of lung injuries linked to vaping, in which eight people have died and more than 500 have become ill.
Most, but not all, have reported "cannabis vaping", in which the vapour contains THC (the psychoactive component in cannabis).
There was a similar case in the UK last year, reported in the British Medical Journal, in which a woman developed lipoid pneumonia and the only explanation doctors could come up with was vaping.
The CDC recommends people "consider refraining from using e-cigarette or vaping products".
The situation in the US has led to President Donald Trump announcing a ban on flavoured e-cigarettes and the supermarket giant Walmart stopping selling them entirely.
Meanwhile, India has announced a ban including three-year jail terms for offenders.
The stance from health bodies in the UK has remained that vaping is 95% safer than smoking.
Deborah Arnott, the chief executive of ASH, said: "Vapers should not be scared back to smoking by the news of vaping illness in the US. Nor should smokers stick to smoking rather than switch to vaping.
"It is essential, however, to only use legal vapes bought from reputable suppliers in the UK and not source illicit unregulated products over the internet."
The World Health Organization says e-cigarettes are "undoubtedly harmful and should therefore be subject to regulation".
It also raises concerns vaping is being aggressively marketed at young people - particularly through the use of flavourings - and risked re-normalising smoking.
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