With tobacco smoke claiming a life every six seconds, the tar-free, electronic alternative could help prevent much of the cancer, heart and lung disease and stroke caused by the toxins in traditional cigarettes, the 50-odd experts wrote to World Health Organisation chief Margaret Chan.
E-cigarettes "could be among the most significant health innovations of the 21st century, perhaps saving hundreds of millions of lives," the group said.
They urged "courageous leadership" from the WHO in guiding global and national approaches to e-cigarettes, which are banned in some countries like Brazil and Singapore and face increasingly strict restrictions in other countries amid uncertainty about their long-term health effects.
The group fears the WHO plans to lump the battery-powered devices, which release nicotine in a vapour instead of smoke and contain fewer toxins, with traditional cigarettes under its tobacco control policy.
This would compel member countries to ban advertising and use of the gadgets in public places, and to impose sin taxes.
"It would be unethical and harmful to inhibit the option to switch to tobacco harm-reduction products" like e-cigarettes, said the letter, a copy of which was given to AFP.
The WHO is working on recommendations for e-cigarette regulation, to be presented to a meeting of member governments in October.
But it does so in a scientific vacuum on the device's long-term safety and its true value as an aid to kicking the tobacco habit.
Some fear its use and often unrestricted promotion could glamorise an addictive habit, and hook non-smoking teenagers on nicotine.
- Nicotine hooks, tar kills -
An estimated seven million people in Europe alone use e-cigarettes, invented in China in 2003.
Addiction specialist Gerry Stimson, an emeritus professor at University College London who co-signed the letter to Chan, said they have been shown to release "very, very fractional levels" of toxins compared to conventional ones.
"People smoke for the nicotine and die of the tar," he told AFP in Paris.
"If you separate the nicotine from the burning of vegetable matter people can still use nicotine but they're not going to die from smoking."
If it listed e-cigarettes as a tobacco product, the WHO would "preserve the position of cigarettes because it makes it harder or more difficult or less desirable to use e-cigarettes," he argued.
The group of epidemiologists, oncologists, addiction experts and health policy specialists who signed the letter included Nigel Gray, a member of the WHO's special advisory committee on tobacco regulation, Michel Kazatchkine, a UN special envoy for AIDS and "harm reduction" advocate, and African Medical Association president Kgosi Letlape.
- Jury still out -
A recent study of nearly 6,000 people who quit smoking in England between 2009 and 2014 found they were 60 times more likely to succeed using e-cigarettes than nicotine patches or gum, or going cold turkey.
However, a study in the Journal of the American Medical Association in March said e-cigarettes "did not significantly predict quitting one year later".
In this grey zone, US regulators have proposed the first restrictions on its soaring $2 billion (1.5 billion-euro) e-cigarette market, with a minimum age limit and health warning labels.
New York has banned them from restaurants, bars, parks, beaches and other public places.
EU lawmakers have agreed to allow countries to restrict e-cigarette sales to pharmacies.
The WHO says tobacco kills nearly six million people a year, and climbing.
On its website, however, it says the world's estimated 1.3 billion smokers should be "strongly advised" not to turn to e-cigarettes until proven safe.
The nicotine in e-cigarettes is typically contained in a propylene glycol liquid that is heated to create a vapour inhaled like smoke.
Some e-liquids are free of nicotine, an addictive stimulant which can be toxic in large amounts.
The WHO would not comment on the contents of the letter.
"We are 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 on health," the agency told AFP by email.
To mark World No Tobacco Day on May 31, it urged countries to raise tobacco taxes, saying a 50-percent increase would reduce the number of smokers by 49 million within next three years and save 11 million lives.