The British Medical Association wants them banned - but more than a million people are now using electronic cigarettes.
And a poll of nearly 1,000 people for BBC Breakfast suggests most support their use in public places and don't want to see a ban.
At Westfield shopping centre in London, Katherine Devlin, who represents the electronic cigarette industry, is vaping - not smoking.
What looks like smoke coming out of her mouth is actually largely water vapour. (click for more)