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Smokers who use e-cigarettes to quit are more likely to succeed than those who use willpower alone or buy nicotine replacement therapies, such as patches or gum, a study suggests.

The survey of nearly 6,000 smokers found a fifth had quit with the aid of e-cigarettes.

That was 60% higher than those who did not use the devices, the study said.

The University College London team said they were "cautiously positive" about the role e-cigarettes could play.

E-cigarette use has shot up in recent years.

Action on Smoking and Health estimate there are more than 2m people using them - triple the number from two years ago.

Half of smokers have tried them compared to 8% in 2010.

Users experience the sensation of smoking by inhaling a vapour which contains a concentration of nicotine.

But they remain controversial. The Welsh government wants to restrict their use in public places, because of fears they normalise smoking.

'Widespread appeal'

However, lead researcher Prof Robert West, one of the UK's leading experts in this field, said: "E-cigarettes could substantially improve public health because of their widespread appeal and the huge health gains associated with stopping smoking."

But he also pointed out that despite the findings - published in the journal Addiction - by far the most effective way of quitting was to use NHS stop smoking services which tripled the odds of a smoker quitting when compared to buying nicotine replacement treatments without specialist help.

And he added: "Some public health experts have expressed concern that widespread use of e-cigarettes could 're-normalise' smoking. However, we are tracking this very closely and see no evidence of it.

"Smoking rates in England are declining, quitting rates are increasing and regular e-cigarette use among never smokers is negligible."

E-cigarettes are currently not available on the NHS, but the drugs regulator the Medicine and Healthcare Products Regulatory Agency is looking to licence them as medicines from 2016.

Prof West said it was too early to say whether and which e-cigarette products could be made available, as there needed to be more research into the safety of long-term use.

However, he said from what is currently known the contents of the e-cigarette vapours will be much less risky than smoking.

A Department of Health spokeswoman said: "E-cigarettes are not risk free, but they carry a lower risk to health than smoking tobacco and may help people who want to stop smoking.

"Any e-cigarette products that are licensed as medicines can be made available on the NHS. We will continue to closely monitor all emerging research."