The number of people who use electronic cigarettes in the UK has tripled over the past two years to 2.1 million, a health charity estimates.
It says just over half of current or ex-smokers have now tried electronic cigarettes, compared with 8% in 2010.
Action on Smoking and Health (Ash) surveyed more than 12,000 adult smokers.
A separate study found that most e-cigarette users were using them to reduce smoking.
Use of e-cigarettes among people who have never smoked remains small at 1%, Ash said.
Ash has commissioned a series of surveys on electronic cigarette use since 2010, with the latest survey conducted in March.
Of those now estimated to be using electronic cigarettes, around 700,000 are thought to be ex-smokers and 1.3 million to be using them alongside normal cigarettes or tobacco.
Current smokers using the cigarettes regularly have risen from 2.7% in 2010 to 17.7% in 2014.
When ex-smokers were asked why they used electronic cigarettes, 71% said they wanted help giving up smoking. Among smokers, 48% said wanted to reduce the amount of tobacco they smoked and 37% said they used e-cigarettes to save money.
Smoking rate fall
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."
Another study, The Smoking Toolkit Study, which covers England, has found that electronic cigarettes are overtaking the use of nicotine products such as patches and gum as an aid to quitting smoking.
It also found that the proportion of smokers who gave up smoking in the past year had increased and smoking rates in England were continuing to fall.
Study leader Prof Robert West 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."
Ms Arnott added: "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."
Ash's survey suggests that most electronic cigarettes users, or "vapers", use a rechargeable product with replaceable cartridges or a reservoir.
Simon Clark, director of Forest, a group that supports smokers, said it welcomed the rise of e-cigarettes and was glad people had a choice of what to smoke.
But he suggested that most smokers using e-cigarettes were experimenting with them rather than using them to give up smoking altogether.
"We haven't seen a significant fall in smokers. Most smokers still find electronic cigarettes quite basic and it will take a few more years for the technology to improve."