A report from a Government-funded agency saying e-cigarettes are 95% less harmful than smoking was based on poor quality evidence, with some links to the tobacco industry, researchers have said.
Experts writing in the British Medical Journal (BMJ) joined The Lancet in criticising the evidence used by Public Health England (PHE) in its report on e-cigarettes.
PHE published its "landmark" report last month, describing it as a "comprehensive review of the evidence".
But several researchers have questioned the robustness of the data and pointed to links between some experts, the tobacco industry and firms that manufacture e-cigarettes.
An estimated 2.6 million people in the UK use e-cigarettes. They are to be licensed and regulated as an aid to quit smoking from 2016.
An editorial in The Lancet medical journal attacked the "extraordinarily flimsy foundation" on which PHE based its major conclusion.
Writing in the BMJ on Tuesday, two further researchers also questioned whether the claims were "built on rock or sand".
Martin McKee, professor of European public health at the London School of Hygiene and Tropical Medicine, and Simon Capewell, professor of clinical epidemiology at the University of Liverpool, said: "A fundamental principle of public health is that policies should be based on evidence of effectiveness."
The pair said a review of evidence included in the PHE report featured only two randomised controlled trials and concluded the evidence was of "low or very low quality".
They said the public would expect PHE's claims that "the current best estimate is that e-cigarettes are around 95% less harmful than smoking" would be based on a detailed review of evidence and modelling.
"In fact, it comes from a single meeting of 12 people convened to develop a multi-criteria decision analysis (MCDA) model to synthesise their opinions on the harms associated with different nicotine-containing products; the results of the meeting were summarised in a research paper."
Prof McKee and Prof Capewell said one sponsor of the meeting was a company called EuroSwiss Health, whose chief executive was reported to have previously received funding from British American Tobacco for an independent study.
He also endorsed British American Tobacco's public health credentials in a sustainability report, they said.
One of the 12 people at the meeting declared funding from an e-cigarette manufacturer but not the funding he is reported to have received previously from tobacco company Philip Morris International, they added.
"The rationale for selecting the members of the panel is not provided, but they include several known e-cigarette champions, some of whom also declare industry funding in the paper.
"The meeting was also attended by the tobacco lead at PHE."
The research paper "tellingly concedes" there is a lack of "hard evidence for the harms of most products on most of the criteria", Prof McKee and Prof Capewell wrote.
"However, none of these links or limitations are discussed in the PHE report."
The research paper from the MCDA meeting was published in the journal European Addiction Research.
In the disclosure statement at the end of the paper, one author is listed as serving as a consultant for "most companies with an interest in tobacco dependence treatments".
Another has served as a consultant to manufacturers of smoking cessation products, while another has received lecture fees from Pfizer and GlaxoSmithKline, a research grant from Pfizer, served as a consultant for Pfizer, the Global Health Alliance for treatment of tobacco dependence, and Arbi Group Srl, an e-cigarette distributor.
The editors of the journal said they were aware that one of the authors "has connections with a company that is associated with one of the largest tobacco industries in the world (British American Tobacco: Nicoventures)".
They said this stand-alone company produces smoking cessation products such as e-cigarettes, and there was no reason to exclude the paper from the journal.
A joint statement from 12 organisations including PHE, the Association of Directors of Public Health, Cancer Research UK, the Faculty of Public Health and the Royal College of Physicians, said: "We all agree that e-cigarettes are significantly less harmful than smoking.
"One in two lifelong smokers dies from their addiction. All of the evidence suggests that the health risks posed by e-cigarettes are relatively small by comparison but we must continue to study the long-term effects."
It said e-cigarettes were the most popular quitting tool in the country, with more than 10 times as many people using them than using local stop smoking services.
"But, we also know that using local stop smoking services is by far the most effective way to quit.
"What we need to do is combine the most popular method with the most effective and that is why we are encouraging those who want to use e-cigarettes to quit smoking to seek the help of their local stop smoking service."