Smoking is still the largest cause of preventable death in the United States, with more than 480,000 people a year dying of smoking-related illnesses. In past decades, smoking was spread throughout society, but today in Britain and the United States, smokers tend to be poor, less educated, or mentally ill.

E-cigarettes, now a multibillion-dollar market, have gained popularity faster than the federal government has managed to regulate them. The Food and Drug Administration has not yet published final rules that would subject them to federal oversight.

A spokesman for the C.D.C. said the agency would not comment on any report other than its own. He reiterated the C.D.C. position on e-cigarettes: “There is currently no conclusive scientific evidence supporting the use of e-cigarettes as a safe and effective cessation tool at the population level. The science thus far indicates most e-cigarette users continue to smoke conventional cigarettes.”

But the report cites evidence from Britain that e-cigarettes have helped with quitting. Robert West, director of Tobacco Studies at University College London, analyzed monthly household survey data going back to 2009 in England, and concluded that use of an e-cigarette during an attempt to quit was associated with a 50 percent increase in the chances of success, compared with using no aid or using a product such as nicotine patches without any professional counseling. He estimated that around 20,000 smokers in England quit smoking in 2014 because of e-cigarettes.

Professor Glantz cited his recent analysis as evidence that e-cigarettes in fact reduce the chances someone will quit smoking.

The Royal College of Physicians is a respected doctors’ group that helps set medical standards in Britain. It issued a groundbreaking report on the dangers of smoking in 1962 that was seen as the precursor of the American surgeon general’s report of 1964 that linked smoking with cancer. The organization’s last major report on reducing harm from tobacco use came out in 2007, before e-cigarettes became mainstream, and its authors said the new one was an attempt to evaluate their benefits and harms.

The report walks through a decade of science, listing studies that find in favor of e-cigarettes as well as studies that do not. It asserts that e-cigarettes are only 5 percent as harmful as traditional cigarettes, a conclusion that some American experts say has been lost in the United States in the rush to condemn e-cigarettes. It states bluntly that long-term effects of nicotine are likely to be minimal.

“The emergence of e-cigarettes has generated a massive opportunity for a consumer as well as a health care-led revolution in the way that nicotine is used in society,” the report said. As the technology of e-cigarettes improves, “so the vision of a society that is free from tobacco smoking, and the harm that smoking causes, becomes more realistic.”