"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."
