E-cigarettes should be banned indoors, the World Health Organisation said.
The health body said there must be no more claims that the devices can help smokers quit - until there is firm evidence to back this - and sales to children should stop.
WHO experts warned the products may pose a threat to youngsters and the foetuses of pregnant women.
It said legal steps need to be taken to end the use of e-cigarettes indoors - both in public spaces and in work - because of fears over second-hand smoke.
The WHO warned exhaled e-cigarette vapour could increase the background air levels of some toxicants and nicotine.
E-cigarettes, officially known as electronic nicotine delivery systems (ENDS), carry a risk to those standing around users, a report for the UN organisation said.
It said: “The fact that ENDS exhaled aerosol contains on average lower levels of toxicants than the emissions from combusted tobacco does not mean that these levels are acceptable to involuntarily exposed bystanders.
“In fact, exhaled aerosol is likely to increase above background levels the risk of disease to bystanders, especially in the case of some ENDS that produce toxicant levels in the range of that produced by some cigarettes.”
The WHO also said the products could spark wider cigarette use in children. Its experts called for a ban on ads that could encourage children and non-smokers to use the devices.
But some researchers suggested tough regulations could prevent smokers accessing products that are potentially less harmful than conventional cigarettes.
Hazel Cheeseman, of charity Action on Smoking and Health, said there was no evidence of any harm to bystanders and warned regulation needed to be proportionate.
She added: “Smoking kills 100,000 people in the UK alone. Smokers who switch to using electronic cigarettes in whole or in part are likely to substantially reduce their health risks.
“Although we cannot be sure that electronic cigarettes are completely safe, as the WHO acknowledges, they are considerably less harmful than smoking tobacco and research suggests that they are already helping smokers to quit.”
A Department of Health spokeswoman said: “More and more people are using e-cigarettes and we want to make sure they are properly regulated so we can be sure of their safety.
“We have already set out our intention to change the law to ban the sale of e-cigarettes to children under 18.
“We are also bringing in new European rules to cover lower strength products which will ban most advertising, limit nicotine levels and set standards for ingredients, labelling and packaging.”
The WHO’s recommendations were published ahead of a meeting involving all countries that have signed up to an international convention on tobacco control. New global guidelines will be agreed at the October meeting.