Usage of electronic cigarettes has tripled over the past two years, with more than two million adults now regularly smoking them, a survey has revealed.
Figures released by health charity ASH show that the number of adults in Britain using the devices has risen from an estimated 700,000 in 2012 to 2.1 million this year.
Nearly two-thirds of users said they also smoked regular cigarettes, with the other third being ex-smokers, an increase in the proportion of former smokers compared to previous years.
Just 1% of those asked who never smoked said they had tried electronic cigarettes.
The YouGov survey found that more than half of ex-smokers (51.7%) say that they have tried electronic cigarettes, compared with just 8.2% in 2010.
It showed there has been a consistent rise in the number of current or former smokers who use electronic cigarettes on a regular basis - up from 2.7% in 2010 to 17.7% this year.
Just over a third (35%) of British adults believe that electronic cigarettes are good for public health while just under a quarter (22%) disagree, the survey said.
The main reason given by ex-smokers for using electronic cigarettes were "to help me stop smoking entirely" (71%) and "to help me keep off tobacco" (48%).
And the biggest reason for current smokers was to "help me reduce the amount of tobacco I smoke, but not stop completely" (48%) followed by "to save money compared with smoking tobacco" (37%) and "to help me stop smoking entirely" (36%).
For the first time, the Ash YouGov survey also asked about the type of electronic cigarette commonly used, with just under half (47%) using rechargeable e-cigarettes with pre-filled cartridges and 41% using rechargeable devices with a separate tank. Just 8% said they most often use disposable e-cigarettes.
Deborah Arnott, chief executive of Ash said: "The dramatic rise in use of electronic cigarettes over the past four years suggests that smokers are increasingly turning to these devices to help them cut down or quit smoking. Significantly, usage among non-smokers remains negligible.
"While it is important to control the advertising of electronic cigarettes to make sure children and non-smokers are not being targeted, there is no evidence from our research that e-cigarettes are acting as a gateway into smoking."
The YouGov survey questioned 12,269 adults online last month.
A separate ongoing survey - the Smoking Toolkit Study carried out in England - has also found that smokers are increasingly using electronic cigarettes as an aid to quitting, overtaking use of medicinal nicotine products such as patches and gum.
The proportion of smokers who have quit in the last year has increased and smoking rates in England are continuing to fall.
Professor Robert West, who led the study, said: "Despite claims that use of electronic cigarettes risks renormalising smoking, we found no evidence to support this view. On the contrary, electronic cigarettes may be helping to reduce smoking as more people use them as an aid to quitting."
Charles Hamshaw-Thomas, legal and corporate affairs director of e-cigarette company E-Lites, said: "Study after study is showing that scaremongering that e-cigarettes are luring people into tobacco is baseless nonsense. The reverse is going on - smokers are switching into e-cigarettes as the way to reduce the harm from tobacco.
"As a result tobacco sales are falling in the UK. And so too are sales of nicotine replacement products (gum and patches) which, as academic research shows, have had limited success."
He added: "The phenomenal growth of e-cigarettes as the most popular alternative to smoking is accelerating the delivery of government targets on reducing tobacco consumption.
"The big e-cigarette companies like E-Lites are keen to work together with government to draw up codes on advertising, bans on sales to children, product quality and safety and continuing research."