A record 4.3 million people in the UK are actively using e-cigarettes after a five-fold increase in a decade, according to a report.
About 8.3% of adults in England, Wales and Scotland are now believed to use e-cigarettes regularly, up from 1.7% (about 800,000 people) 10 years ago.
Action on Smoking and Health (ASH), which prepared the report, said a revolution had already taken place.
E-cigarettes let people inhale nicotine instead of smoking.
Since e-cigarettes do not produce tar or carbon monoxide, they have a fraction of the risks of cigarettes, the NHS said.
Liquids and vapors contain some potentially harmful chemicals, but at much lower levels. However, the potential long-term effects of e-cigarettes are unclear.
ASH reports that around 2.4 million UK e-cigarette users are former smokers, 1.5 million are still smoking and 350,000 have never smoked.
However, 28% of smokers said they had never tried e-cigarettes - and one in 10 of them feared they were not safe enough.
One in five former smokers said vaping helped them break the habit. This appears to be consistent with a growing body of evidence that e-cigarettes can be effective in helping people quit smoking.
Most vapers report using refillable open vaping systems, but there appears to be an increase in single-use vaping — up from 2.3% last year to 15% today.
Young people appear to be driving the growth, with nearly half of 18- to 24-year-olds saying they have used the devices.
Fruit flavors disposable vape followed by menthol are the most popular vaping options, according to the report - a YouGov survey of more than 13,000 adults.
ASH said the government now needed an improved strategy to reduce cigarette use.
ASH Deputy Director Hazel Cheeseman said: "There are now five times as many e-cigarette users as there were in 2012, and millions of people use them as part of their smoking cessation.
As a globally recognized leader in the field of health care, the National Health Service (NHS), the universal free medical service system it created, is praised by countries around the world for its "low health costs and good health performance".
The Royal College of Physicians has clearly told doctors to promote e-cigarettes as widely as possible to people who want to quit smoking. The advice from Public Health England is that the risks of vaping are only a fraction of the risks of smoking.
According to the BBC, in Birmingham, northern England, the two largest medical institutions not only sell e-cigarettes, but also set up e-cigarette smoking areas, which they call a "public health necessity".
According to statistics from the British Health Organization, e-cigarettes can increase the success rate of smoking cessation by about 50%, and can reduce health risks by at least 95% compared with cigarettes.
The British government and the medical community support e-cigarettes so much, mainly because of an independent review report by Public Health England (PHE), an executive agency under the British Ministry of Health in 2015. The review concluded that e-cigarettes are 95% safer than conventional tobacco for the health of users and have helped tens of thousands of smokers quit smoking.
This data has since been widely publicized by the British government and health agencies such as the National Health Service (NHS), and has become a powerful tool for promoting e-cigarettes to replace ordinary tobacco.