Public health specialists in the UK have urged the World Health Organisation (WHO) not to "control and suppress" e-cigarettes as it prepares to publish global guidelines on the devices.
A letter signed by more than 50 researchers and specialists, including Professor Robert West, from University College London, said e-cigarettes have the potential to save millions of lives.
It urged the WHO not to impose regulations on the devices in the same way it does with conventional cigarettes.
The global health advisor is preparing to publish recommendations about e-cigarettes to governments later this year.
The letter, seen by the BBC, said: "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."
The signatories published the letter after claiming to have seen a leaked document from the WHO which labelled the e-cigarettes as a "threat".
A WHO spokesman told the BBC: "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."
Research published last week by Professor West suggested that e-cigarettes could help improve the success rate of people trying to quit smoking compared to nicotine patches or gum.
But others say that not enough is known about the long-term effects of the devices, which deliver nicotine in a vapour.
Smoking is the largest preventable cause of cancer and accounts for more than one in four cancer deaths in the UK, said Alison Cox, Cancer Research UK's head of tobacco policy.
"So helping smokers to stop is a vital contribution to the health of the UK.
"E-cigarettes may have a role in helping people to quit smoking but, while the rapid rise in their popularity suggests a real opportunity, the evidence for their effectiveness is so far limited.
"Cancer Research UK is funding much-needed research into e-cigarette use to help inform policy development and individuals' choices, and look forward to seeing both the WHO recommendations and the updated NICE guidelines on reducing the harm caused by tobacco," she added.
"Electronic cigarettes for smoking cessation: a randomised controlled trial." Dr Christopher Bullen, et al. The Lancet, Volume 382, Issue 9905, Pages 1629 - 1637, 16 November 2013 DOI: 10.1016/S0140-6736(13)61842-5