Smokers hoping to quit using e-cigarettes may find it harder than they think, claim US researchers.

They found smokers who used e-cigarettes were 49 per cent less likely to decrease cigarette use and 59 per cent less likely to quit smoking compared to smokers who never used e-cigarettes.

The study was carried out by researchers at the University of California, San Diego School of Medicine.

Controversy continues over whether e cigarettes have net benefits, amid fears that using them in public places will ‘re-normalise’ smoking, especially among young people, and reverse declining smoking rates.

Smokers who used e-cigarettes were 49 per cent less likely to decrease cigarette use, researchers found

But a major scientific review last year found smokers who use e cigarettes are more likely to stop or reduce their smoking.

Battery-powered e-cigarettes are devices allowing users to inhale nicotine while avoiding the harm caused by tobacco smoke.

It is estimated that two thirds of ‘vapers’ are current smokers, many of whom are trying to quit, and most of the remainder are ex-smokers.

The UK’s drug watchdrug has decided they must be regulated as medicines to make the products ‘safer and more effective’ - but this will not happen until 2016.

Dr Wael Al-Delaimy, professor and chief of the Division of Global Public Health in the Department of Family Medicine and Public Health, who led the new study, said: ‘Based on the idea that smokers use e-cigarettes to quit smoking, we hypothesised that smokers who used these products would be more successful in quitting.

‘But the research revealed the contrary. We need further studies to answer why they cannot quit. One hypothesis is that smokers are receiving an increase in nicotine dose by using e-cigarettes.’

The new study which followed 1,000 California smokers over the course of one year found daily smokers and women were more likely to have tried e-cigarettes.

E-cigarette smokers were also 59 per cent less likely to quit compared to those who never used the devices

The study was published in the American Journal of Public Health (must credit).

Dr Al-Delaimy believes the study will inform the United States Food and Drug Administration and other regulators as they create guidelines for e-cigarettes.

In December the Cochrane Library, the world’s leading producer of systematic reviews, carried out a review which showed

almost one in 10 smokers using e cigarettes had been able to quit the habit up to a year later, and around one-third had cut down.

British experts were sceptical about the new study’s findings.

Prof Linda Bauld, Professor of Health Policy, University of Stirling, said: ‘This survey of 1,000 smokers in California claims that 'smokers who have used e-cigarettes may be at increased risk of not being able to quit smoking'.

‘Yet the study was not designed to determine this.

Controversy continues over e cigarettes, amid fears that using them in public places will 're-normalise' smoking, especially among young people, and reverse declining smoking rates (file picture)

‘It merely asked smokers at baseline and follow up if they'd used e-cigarettes with no information on whether this was trying them once or twice or regular use.

‘It then simply described whether people who reported ever use had stopped smoking tobacco.

‘This was not a study of the effectiveness of e-cigarettes in helping people quit smoking and should not be interpreted as such.’

Professor Peter Hajek, Director of the Tobacco Dependence Research Unit, Queen Mary University of London, said: ‘The survey provides little useful information.

‘Smokers who found e-cigarettes effective and stopped smoking were excluded and only smokers who did not find e-cigarettes helpful were followed-up.