The use of electronic 'E-cigarettes' has tripled over the last two years, with over 2 million Britons now regularly smoking them.
Health charity ASH released figures showing a rise from 700,000 in 2012 to 2.1 million this year.
The group said two thirds of e-cigarette smokers still smoke normal cigarettes, while the remaining third were former smokers.
More top news
Unpaid internships should be banned to prevent young people missing out on jobs in top professions, a new report has said.
The study by the National Childbirth Trust and National Federation of Women's Institutes found a lack of staff left some women frightened.
Extracts released by 10 Downing Street suggest Mrs May will say the UK will not settle for a "half-in, half-out" policy.