GPs should warn smokers there is currently little evidence on the long-term benefits or harms of e-cigarettes, health officials have said.
The move from the National Institute for Health and Care Excellence (Nice) comes despite backing for e-cigarettes from Public Health England (PHE), which says they are a useful aid for quitting.
In new draft guidance, Nice does not list e-cigarettes as a recommendation to help people quit but says doctors and nurses should have a conversation with their patients about their use.
They should tell people that although e-cigarettes are not licensed medicines, they are regulated by law and some smokers have found them helpful when they wish to quit smoking.
It says patients should be told, though, that there "is currently little evidence on the long-term benefits or harms of these products".
Nevertheless, staff should "be aware that Public Health England and the Royal College of Physicians have stated that e-cigarettes are significantly less harmful to health than tobacco".
E-cigarettes are not available on prescription on the NHS.
The guideline says GPs and nurses should recommend a combination of behavioural support from NHS Stop Smoking services and products such as nicotine replacement therapies as an aid to quitting.
Nice also recommends employers "negotiate a smoke-free workplace policy with employees" and form rules around cigarette breaks for staff.
Workplace policies should "state whether or not smoking breaks may be taken during working hours and, if so, where, how often and for how long," it said.
In December, the US surgeon general issued a stark warning over the risks of e-cigarettes - putting him at odds with UK public health officials.
America's most senior doctor Vivek Murthy said e-cigarette use among young people and young adults "is not safe" and is "now a major public health concern".
He said the negative health effects and potentially harmful doses of heated chemicals in e-cigarette liquids are not completely understood.
However, a PHE report in 2015 said e-cigarettes should not be viewed in the same way as smoking.
It said "best estimates show e-cigarettes are 95% less harmful to your health than normal cigarettes, and when supported by a smoking cessation service help most smokers to quit tobacco altogether".
The report - which was heavily criticised - said any new regulations should "maximise the public health opportunities of electronic cigarettes".
It added: "While vaping may not be 100% safe, most of the chemicals causing smoking-related disease are absent and the chemicals which are present pose limited danger."
According to Nice, smoking is the main cause of preventable illness and premature death in England.
In 2014/15, an estimated 475,000 NHS hospital admissions in England were linked to smoking and 17% (78,000) of all deaths in 2014 were attributed to smoking.
Treating smoking-related illness is estimated to cost the NHS £2.5 billion a year while the wider cost to society is about £12.7 billion.
Professor Gillian Leng, deputy chief executive at Nice, said: "The guideline committee found little evidence of the long-term effects of e-cigarettes on human health and subsequently their use was not included as a recommended aid to stop smoking.
"However, Nice is keen to hear feedback on the use of e-cigarettes during the consultation on this guideline.
"We acknowledge that Public Health England and the Royal College of Physicians have stated that e-cigarettes are significantly less harmful to health than tobacco.
"This has been included in the guideline along with the recommendation that patients should receive advice on e-cigarettes so that they can make informed decisions on how to stop smoking."