Around 2.9 million adults in the UK are currently using electronic cigarettes.
Around half (1.5 million) are ex-smokers who have abandoned tobacco in favour of e-cigarettes.
Experts say e-cigarettes carry a fraction of the health risks of ordinary cigarettes and can be an effective way of stopping smoking, particularly when combined with extra quitting support.
However, the devices have stoked controversy ever since they were introduced to the domestic market in 2005.
Opponents say the nicotine they deliver is harmful and that e-cigarettes could act as a 'gateway' to smoking for children and teenagers.
However UK health authorities have repeatedly said that e-cigarettes are helping to cut smoking rates and that there is no evidence that they lure youngsters into a tobacco smoking habit.
The evidence base is changing, and more recently another nicotine-delivery product has appeared on the market in which tobacco is heated in a cigarette-like device, rather than burned, in order to deliver a more convincing smoking-like 'hit'.
E-cigarettes, or the latest 'heated tobacco products', are devices for delivering nicotine vapour instead of tobacco smoke. What they share between them is that nicotine is vaporised rather than being a product of burning.
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