E-cigarettes are now the most popular form of support to stop smoking, overtaking nicotine gum and skin patches, researchers have said.
More than one million smokers in England used an electronic cigarette in a bid to kick the habit last year, according to academics at University College London (UCL).
They estimated that 2.6 million of England's eight million smokers tried to quit last year. Two fifths of those who tried to quit used an e-cigarette compared with 26% who used a licensed nicotine product in their attempt.
"E-cigarettes have overtaken more traditional methods as the most widely used support for smokers wanting to quit," said Robert West, professor of health psychology at UCL.
"Their impact on public health at present comes from attracting people who would otherwise have tried to stop without any useful form of support.
"We estimate that e-cigarettes have probably helped around 20,000 smokers to quit each year, that wouldn't have otherwise."
Mike Knapton, associate medical director at the British Heart Foundation, added: "Although e-cigarettes are much less harmful than smoking cigarettes, there is no doubt that more research is needed into the potential long term effects of the use of them.
"This unique study shines a light on just how popular e-cigarettes have become as an aid for smokers trying to quit and we need to listen to what is helping people the most on their path to a smoke free life."
Meanwhile, Cancer Research UK (Cruk) called on the Government to force big tobacco companies to pay a penny levy on every single cigarette sold in the UK.
Cruk said the move would raise an extra £500 million to invest into public health services and advertising campaigns.
It is submitting a petition of more than 16,000 signatures to Parliament supporting the move today - No Smoking Day.
The charity said the decline in smoking rates is "stalling".
"Decades of work have gone into reducing the number of people who will be affected by a tobacco related illness," said Alison Cox, Cruk's director of cancer prevention.
"There's been great progress, but unless more is done, another generation of lives will be devastated by smoking.
Labour MP Sir Kevin Barron, vice-chairman of the All Party Parliamentary Group on Smoking and Health, said: "Smoking still kills 100,000 people in the UK each year and increasingly the burden falls on the poorest communities.
"The Government can't be complacent and must set bold ambitions to achieve a tobacco free future, matched with the necessary funding to make this happen.
"Making the tobacco industry pay for vital stop smoking services and mass media campaigns through a levy will help reduce the number of people smoking and save lives."