Media playback is unsupported on your device Media caption Some doctors argue it is too early to know whether benefits outweigh potential risks, as Jane Dreaper reports

A letter signed by more than 50 researchers and public health specialists is urging the World Health Organization (WHO) to "resist the urge to control and suppress e-cigarettes".

The letter says the devices - which deliver nicotine in a vapour - could be a "significant health innovation".

But the UK's Faculty of Public Health says it is too early to know whether benefits outweigh potential risks.

The WHO said it was still deciding what recommendations to make to governments.

Media playback is unsupported on your device Media caption BAT's Kingsley Wheaton: "We've made major investments in e-cigarettes"

The open letter has been organised in the run-up to significant international negotiations on tobacco policy this year.

Supporters of e-cigarettes, who argue the products are a low-risk substitute for smoking, fear they might become subject to reduction targets and advertising bans.

1. On some e-cigarettes, inhalation activates the battery-powered atomiser. Other types are manually switched on

2. A heating coil inside the atomiser heats liquid nicotine contained in a cartridge

3. Liquid nicotine becomes vapour and is inhaled. The 'smoke' produced is largely water vapour. Many e-cigarettes have an LED light as a cosmetic feature to simulate traditional cigarette glow.

There has been a big growth in the market for e-cigarettes, but the Department of Health says they are not risk-free.

Critics say not enough is known about their long-term health effects. A recent report commissioned by Public Health England said e-cigarettes required "appropriate regulation, careful monitoring and risk management" if their benefits were to be maximised.

The letter has been signed by 53 researchers - including specialists in public health policy and experts such as Prof Robert West, who published research last week suggesting that e-cigarettes are more likely to help people give up smoking than some conventional methods.

We are deeply concerned that classifying these products as tobacco will do more harm than good Open letter to the WHO

Some of the signatories work on research into tobacco science and smoking cessation. Three were involved in advising the National Institute for Health and Care Excellence (NICE) on its guidelines about reducing the harm from tobacco.

'Perverse effect'

The letter says: "These products could be among the most significant health innovations of the 21st Century - perhaps saving hundreds of millions of lives.

"If regulators treat low-risk nicotine products as traditional tobacco products... they are improperly defining them as part of the problem.

"Regulators should avoid support for measures that could have the perverse effect of prolonging cigarette consumption.

"We are deeply concerned that the classification of these products as tobacco will do more harm than good.

"The potential for tobacco harm reduction products to reduce the burden of smoking-related disease is very large."

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The organisers of the letter quote a leaked WHO document that refers to e-cigarettes as a "threat... which could result in a new wave of the tobacco epidemic".

The WHO treaty on tobacco control currently covers 178 countries and 90% of the world's population.

E-cigarettes are battery-powered devices that mimic the experience of smoking. Users inhale a vapour from a heated liquid that contains a concentration of nicotine.

Prof West, of University College London told the BBC e-cigarettes should be "regulated appropriate to what they are" and that they are "orders of magnitude safer" than tobacco cigarettes.

He called for "bespoke regulation", including banning sales for under-18s and having marketing directed at those who already smoke.

We need to weigh up the benefits of fewer people smoking against the risk of electronic cigarettes leading to more people starting to smoke Prof John Ashton, Faculty of Public Health

A WHO spokesman said: "WHO is currently working on recommendations for governments on the regulation and marketing of e-cigarettes and similar devices.

"This is part of a paper that will be submitted to the parties of the WHO Framework Convention on Tobacco Control later this year.

"We are also working with national regulatory bodies to look at regulatory options, as well as toxicology experts, to understand more about the possible impact of e-cigarettes and similar devices on health."

The British Medical Association (BMA) has called for stronger regulation of the devices in the UK.

Dr Vivienne Nathanson, the BMA's director of professional activities, told BBC Breakfast there was evidence that children who had never smoked were starting to use e-cigarettes, having been influenced by marketing campaigns.

"Rather like cigarettes in the 50s and 60s, we really need to look at that and, I believe, ban it (advertising), to stop them advertising in a way that attracts children," she added.

Prof John Ashton, president of the Faculty of Public Health, said he was also concerned about children using e-cigarettes.

'No uniformity'

"We need to weigh up the benefits of fewer people smoking against the risk of electronic cigarettes leading to more people starting to smoke, particularly children," he said.

Prof Martin McKee, of the London School of Hygiene and Tropical Medicine, said: "The health community is completely divided on the subject of whether electronic cigarettes are safer than real cigarettes.

"While the signatories to this letter are clearly supportive, the World Health Organization, correctly, bases its decisions on the best available evidence."

He said it would be "premature" to advocate the use of e-cigarettes until their safety had been established.

The Welsh government wants to restrict the use of e-cigarettes in enclosed public places, because of concerns that they normalise smoking.

Richard Evans, vice-chairman of the Welsh Pharmacy Board, which is part of the Royal Pharmaceutical Society in Wales, believes they should be regulated in the same way as traditional cigarettes.

He said: "At the moment the products that are on the market - we don't know what standard they are. They can vary from product to product - there is no uniformity at all.