Vaping has been approved by health experts after the first long-term study of its effects in ex-smokers.
Scientists found those who switched from real to e-cigarettes had far fewer toxins and cancer-causing substances in their bodies than continual smokers after six months.
While participants who failed to make a clean break still had significant amounts of tobacco-related toxins in their saliva and urine.
The findings come after a Cancer Research UK-funded study involving 181 people smokers and ex-smokers who had used e-cigarettes or nicotine replacement therapy (NRT) products such as patches and nasal sprays for at least six months.
Experts hope the results will reassure would-be quitters who have been confused by mixed messages about the safety of e-cigarettes.
Previous studies which suggested vaping is as harmful as smoking have little in common with real-world experience, it is claimed.
Lead author Dr Lion Shahab, from University College London, said: "Our study adds to existing evidence showing that e-cigarettes and NRT are far safer than smoking, and suggests that there is a very low risk associated with their long-term use.
"We've shown that the levels of toxic chemicals in the body from e-cigarettes are considerably lower than suggested in previous studies using simulated experiments. This means some doubts about the safety of e-cigarettes may be wrong.
"Our results also suggest that while e-cigarettes are not only safer, the amount of nicotine they provide is not noticeably different to conventional cigarettes.
"This can help people to stop smoking altogether by dealing with their cravings in a safer way."
Compared with full-time smokers, e-cigarette-only users had 97% lower levels of one toxic chemical, NNAL, that is strongly associated with lung cancer.
But there was little difference in NNAL levels between cigarette-only users and those who both smoked real cigarettes and vaped e-cigarettes.
Other substances called volatile organic compounds (VOCs), including highly carcinogenic acrylamide and cyanide-releasing acrylonitrile, were also far less present in the bodies of e-cigarette users.
Alison Cox, director of cancer prevention at Cancer Research UK, said: "Around a third of tobacco-caused deaths are due to cancer, so we want to see many more of the UK's 10 million smokers break their addiction.
"This study adds to growing evidence that e-cigarettes are a much safer alternative to tobacco, and suggests the long term effects of these products will be minimal.
"Understanding and communicating the benefits of nicotine replacements, such as e-cigarettes, is an important step towards reducing the number of tobacco-related deaths here in the UK."
A spokesman for the UK Vaping Industry Association, said: "Vaping has significantly grown in popularity in the UK in the past few years. There are now nearly three million vapers.
"We urge the Government to recognise that this represents a huge public health opportunity, and work with the vaping industry to encourage more smokers to switch from tobacco to vaping."
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