The number of smokers using e-cigarettes to quit the habit is declining amid "little in the way of official guidance" from the market or the NHS, according to a report.
E-cigarettes are by far the most popular method used to quit smoking in the UK, but their usage has dropped from 69% of smokers or ex-smokers using them in 2014 to 62% last year, analysts Mintel said.
Meanwhile, use of non-prescription nicotine replacement therapy products remains stable at 15%, as does use of nicotine replacement gums or patches on prescription from health professionals at 14%.
The survey found more than half of Britons (53%) believe e-cigarettes should be regulated by the NHS, and 57% are concerned there is not enough information available on how the devices work.
Mintel senior beauty and personal care analyst Roshida Khanom said: "The lack of licensed products positioned as smoking cessation methods is hampering the e-cigarette sector and, as a result, we are not seeing as many new users enter the market.
"Our research shows that the majority of consumers don't know how e-cigarettes work and that they would like to see more NHS regulation.
"Those who are using e-cigarettes as a smoking cessation method are doing so with little in the way of official guidance, whether from the market or the NHS."
Mintel's report follows US surgeon general Vivek Murthy issuing a stark warning over the risks of e-cigarettes in December, putting him at odds with UK public health officials.
Dr Murthy said e-cigarette use among young people and young adults "is not safe" and is "now a major public health concern" as their brands were more vulnerable to the negative consequences of nicotine - the key ingredient in e-cigarettes.
His view contrasts significantly with a report from Public Health England (PHE) which said "best estimates show e-cigarettes are 95% less harmful to health than normal cigarettes, and when supported by a smoking cessation service, help most smokers to quit tobacco altogether".
The Mintel report said 30% of Britons currently smoke regular cigarettes, compared to 33% in 2014.
Younger people aged 25 to 34 are significantly more likely to smoke than those aged over 55, at 47% compared with 18%.
Some 42% of smokers intend to quit in the future, but just over one quarter (27%) have no interest in quitting whatsoever, the report said.
:: Mintel surveyed 879 adult internet users who are currently trying to quit or have quit smoking in October.