The concern electronic cigarettes are a gateway to smoking might be unfounded, the first official UK figures suggest.
Data from the Office for National Statistics indicate those who use e-cigarettes, are almost entirely current or former smokers.
E-cigarettes were mainly used to help smokers quit and because users saw them as being less harmful than cigarettes, the ONS said.
And the proportion of adults who smoked cigarettes had fallen to 19%.
Most of the figures from the ONS are for the year 2013, so it is possible that the picture is still changing.
The proportion of smokers had plummeted from 46% in 1974 to 19% in 2013, the ONS said.
Not only had fewer people taken up smoking, but more smokers had quit.
And many smokers and former smokers were using e-cigarettes.
E-cigarettes have been increasing in popularity since they were introduced in Europe in 2006.
Some say they could normalise smoking or be a gateway to smoking by introducing non-smokers to nicotine, while others feel they are a useful way to help get people off tobacco.
The ONS said it had chosen to publish preliminary findings on e-cigarette use because there was a need for more information.
Data collected between January and March 2014 suggests e-cigarettes are used almost exclusively by smokers and former smokers.
About one in 10 current cigarette smokers surveyed and one in 20 of the former smokers said they were now using e-cigarettes.
Over half of e-cigarette users surveyed said their main reason was to stop smoking, and about one in five said it was because they thought they were less harmful than cigarettes.
Prof Kevin Fenton, National Director of Health and Wellbeing at Public Health England, said given the number of smokers now using e-cigarettes people needed to be assured of their quality and safety.
"Balanced and effective regulation of e-cigarettes will help manage the risks and maximise the potential for these products to replace smoking - greatly reducing smoking related disease that kills nearly 80,000 people in England every year."
At present they are not available on the NHS, unlike other smoking cessation aids such as nicotine patches.
Dr Penny Woods, Chief Executive of the British Lung Foundation, said: "These data should again alleviate the fears expressed by some over an e-cigarette gateway effect - people trying e-cigarettes before moving on to the much more harmful practice of smoking. However, this is something we'll need to keep a watchful eye on, as the situation may well change as e-cigarettes continue to become more commonplace."