E-cigarettes are almost twice as effective as nicotine replacements for helping smokers quit, a study suggests.
A trial found 18% of smokers who used them to quit remained smoke-free after a year, compared with 9.9% of those using nicotine-replacement treatments.
The study of 886 smokers is the first to test how effective modern e-cigarettes are for quitting.
Researchers hope their findings will lead to vaping devices being routinely offered by stop-smoking services.
Public Health England has already called for e-cigarettes to be made available on the NHS within five years, pointing to a body of research that suggests they are at least 95% less harmful than cigarettes.
However, up until now there had been a shortage of evidence on how effective they were as stop-smoking tools.
Lead researcher Prof Peter Hajek, from Queen Mary University of London, said: "Although a large number of smokers report that they have quit smoking successfully with the help of e-cigarettes, health professionals have been reluctant to recommend their use because of the lack of clear evidence from randomised controlled trials.
"This is now likely to change."
'Accelerate' smoking reduction
Participants in the trial, who were dependent on smoking and had previously failed to give up, attended NHS stop-smoking services and were randomly assigned into two groups:
those who received a nicotine-replacement treatment of their choice, which included gum, patches, lozenges, sprays and inhalators, or a combination of treatments, for up to three months.
those who got an e-cigarette starter pack with one or two bottles of e-liquid (two to four weeks' supply).
Those given e-cigarettes were encouraged to buy future supplies of their own choice of strengths and flavours, and all participants received weekly one-on-one behavioural support for at least four weeks.
The study, published in the New England Journal of Medicine, found that in addition to having higher rates of quitting, more e-cigarette users reduced their smoking by at least 50%.
E-cigarettes also provided higher satisfaction and were rated as more helpful than nicotine-replacement treatment.
A higher proportion of those who used the devices experienced mouth and throat irritation (65% v 51%), although people using the nicotine-replacement treatments were more likely to report nausea (38% v 31%). These effects were mostly mild.
Prof Hajek said he hoped the results of the study would lead to stop-smoking services offering quitters an e-cigarette starter pack and guidance on how to vape, after which they could pay for their own supplies.
"This may ultimately further accelerate the reduction in smoking and in smoking-related diseases," said Dunja Przulj, another author of the study, also from Queen Mary University of London.
The study has some limitations.
Because people had known which treatment they had received - as opposed to being "blinded" as they are in most randomised controlled trials - it was possible participants may have perceived nicotine replacements as an inferior option and put less effort into quitting, the authors said.
They also said more work was needed to determine if their results would apply to countries outside of the UK and for less dependent smokers.
The study comes after an independent review of evidence on e-cigarettes, by Public Health England, published in February last year, concluded there was "overwhelming evidence" they were far safer than smoking and "of negligible risk to bystanders".
However, some experts have said e-cigarettes, while safer than normal cigarettes, are not harm-free, and their long-term effects are not yet known.
Responding to this latest research, Public Health England said: "All stop-smoking services should welcome smokers who want to quit with the help of an e-cigarette."