More people are vaping than ever before, with a 12.5% growth in one year, research suggests.
An estimated 3.6 million people in the UK are currently vaping (7.1% of the population), up from 3.2 million the year before (6.2% of the population), a large survey of adults showed.
This is up from an estimated 1.3 million (2.7% of people) in 2003.
The peak age range for current e-cigarette use is 35 to 44-year-olds (9.5%) followed by 45 to 54-year-olds (9.3%), and then 25 to 34-year-olds (7.8%).
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The lowest vaping rate stands at 4.3% for young adults aged 18 to 24, followed by those over 55 at 5.6%.
The charity Action on Smoking and Health (Ash), which carried out the research, said the number of vapers is half the 7.2 million smokers in the UK in 2018.
Its poll of over 12,000 people suggests the proportion of vapers who are ex-smokers continues to grow, reaching 54.1% in 2019.
Some 31% of vapers who are ex-smokers said their main reason for using them was to help them quit (31%) followed by aiming to prevent relapse back to smoking (20%), enjoying vaping (14%) and saving money (13%)
Some 0.8% of people who have never smoked are current vapers, while 39.8% of vapers both smoke and vape, though this is falling, the research found.
Of those people who had never smoked but had used e-cigarettes, 73% said they did so to give it a try.
The proportion of smokers believing vaping is less harmful than smoking has fallen from just over a half (54%) last year, to just under a half (48%).
Ann McNeill, professor of tobacco addiction at King's College London, and lead author of the independent evidence reviews of e-cigarettes for Public Health England (PHE), said: "Vaping isn't risk free, but it's much less risky than smoking, which kills nearly 100,000 people a year in the UK."
Deborah Arnott, chief executive of Ash, said: "Although e-cigarettes are now the most popular quitting aid, our survey finds that in 2019 over a third of smokers have still never tried vaping.
"As Stoptober kicks off, we'd encourage smokers who haven't done so yet, to give vaping a try.
"E-cigarettes have been shown to be a very effective aid for smokers trying to quit, either on their own or with help from stop smoking services."
She said US reports of illnesses linked to vaping were "obviously concerning, but it appears to be linked to the misuse of e-cigarettes for illicit drug delivery.
"Nothing like this has been seen in the UK to date, where a proper regulatory system is in place for nicotine containing e-cigarettes, which is not yet the case in the US.
"Vapers should not be scared back to smoking by the news of vaping illness in the US.
"Nor should smokers stick to smoking rather than switch to vaping. It is essential however, to only use legal vapes bought from reputable suppliers in the UK and not source illicit unregulated products over the internet."
In the US, regulators are examining cases where people who have vaped have been found to have oil-like substances in their lungs.
Several deaths have also been reported while overall, more than 450 cases of a lung disease linked to vaping are being investigated.
Walmart has also said it will stop selling e-cigarettes at its stores.