More than two million people in Britain now use electronic cigarettes, although 59% also smoke regular cigarettes alongside, new figures suggest.
Estimates from the Office for National Statistics (ONS) revealed there are 2.2 million current e-cigarette users - 4% of the population.
Just over half (53%) of e-cigarette users said they used vaping as an aid to quitting regular smoking.
The data also suggests there are 3.9 million people who are former users of e-cigarettes and a further 2.6 million people said they had tried an e-cigarette but never went on to use it.
Just over one in five (22%) of current users said their main reason for vaping is because they feel e-cigarettes are less harmful than cigarettes.
One in 10 (9%) said their main reason was because they could use e-cigarettes indoors.
A relatively small number (56,000) of current e-cigarette users have never previously smoked, the data showed.
Of the former e-cigarette users, around three-quarters said they were currently smoking cigarettes. Some 59% of the current users said they also smoked cigarettes (1.3 million).
When it comes to regular cigarette smoking, the numbers who smoke continues to fall.
In 2014, 19% of adults smoked, with 20% of men smoking - the lowest on record.
Some 17% of women smoked, slightly up on the previous year.
Senior ONS statistician Jamie Jenkins said: "These figures continue a long-term trend for fewer people to smoke cigarettes - only 19% of adults today compared with 46% when our survey began in 1976.
"While the majority of people are using e-cigarettes as an aid to quit smoking it seems they don't work for everyone, as three-quarters of former vapers are still smoking cigarettes."
The most common age group for women to vape is 35 to 44. Among men, most users are aged 45 to 64.
Most people (67%) use an e-cigarette on a daily basis and a further 19% use one at least once a week.
People tend to prefer e-cigarettes that do not resemble a cigarette.
Dr Penny Woods, chief executive of the British Lung Foundation, said: "It is encouraging that the number of adult smokers is the lowest on record, but we can't be complacent.
"Smoking cessation services - the most effective way of helping people quit - are under threat around the country due to budget cuts. If we want to continue helping people to quit and look after their lung health, investment in these vital services must be protected.
"While there is still a lack of data on the long-term health impact of e-cigarettes, it is encouraging to note that three-quarters of people now know that vaping is significantly less harmful than smoking.
"E-cigarettes shouldn't be seen as a permanent alternative to smoking, and these figures confirm that they don't work for everyone as a quitting aid.
"However, if you haven't successfully quit using other methods, including your local smoking cessation services, then it may well be worth trying e-cigarettes, with an aim of eventually quitting them too."
Deborah Arnott, chief executive of Action on Smoking and Health, said the slowdown in the numbers of people stopping smoking was worrying and there was a need for a comprehensive tobacco control policy.
She added: "The results on electronic cigarettes show that the majority of users are smokers who are using them to quit smoking. This is encouraging as e-cigarettes are far safer than tobacco products and using them as a quitting aid will significantly reduce the risk of heart disease and cancer caused by smoking."