For the first time, more than half of the current 2.9 million e-cigarette users have since given up smoking, Action on Smoking and Health (Ash) said in its annual survey into the use of electronic cigarettes and vapourisers.
Many vapers still "dual-use" combustible and electronic cigarettes, which means they are still exposed to the cancer causing substances in tobacco smoke, the study added.
Ann McNeill, professor of tobacco addiction at King's College London, said: "This year's Ash survey finds that around 1.5 million vapers are ex-smokers, for the first time a larger number than those who continue to smoke.
"This is encouraging news as we know that vapers who continue to smoke continue to be exposed to cancer-causing substances.
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"The message for the 1.3 million vapers who still smoke is that they need to go further and switch completely."
The survey showed people were over-estimating the dangers to vaping, with only 13 per cent of respondents recognising that e-cigarettes were a lot less harmful than smoking and more than a quarter (26 per cent) thinking they were more or equally harmful.
The most common reasons given by e-cigarette users for ditching the tobacco were to help them give up completely and to save money.
But Ash chief executive Deborah Arnott pointed out that there were still nine million active smokers.
