Consuming e-cigarettes is far safer and less toxic than smoking conventional tobacco cigarettes, according to the findings of a study analysing levels of dangerous and cancer-causing substances in the body.

Researchers found that people who switched from smoking regular cigarettes to e-cigarettes or nicotine replacement therapy (NRT) such as gum or patches for at least six months had much lower levels of toxins in their saliva and urine than those who continued to smoke.

"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," said Lion Shahab, a specialist in epidemiology and public health at University College London who led the work.

E-cigarettes, which heat nicotine-laced liquid into vapour, have grown into an $8 billion-a-year market, according to Euromonitor International - more than three times that of NRT products. They are, however, still dwarfed by a tobacco market estimated by Euromonitor to be worth around $700 billion.

Many health experts think e-cigarettes, or vapes, which do not contain tobacco, are a lower-risk alternative to smoking and potentially a major public health tool.

But some question their long-term safety and worry that they may act as a "gateway" to taking up conventional cigarettes. The U.S. surgeon general in December urged lawmakers to impose price and tax policies that would discourage their use.

 Monday's study, published in the journal Annals of Internal Medicine, analysed saliva and urine samples from long-term e-cigarette and NRT users as well as smokers, and compared levels of key chemicals found in their bodies.

It found that smokers who switched completely to e-cigarettes or NRT had significantly lower levels of toxic chemicals and carcinogens compared to people who continued to smoke tobacco cigarettes.

Those who used e-cigarettes or NRT but did not completely quit smoking did not show the same drop in toxin levels. This underlined that a complete switch was needed to get the long-term health benefits of quitting tobacco, the researchers said.

The World Health Organization says tobacco is the world's biggest preventable killer, with a predicted cumulative death toll of a billion by the end of this century if current trends continue. Tobacco smoking currently kills around 6 million people a year.

Kevin Fenton, national director of health and wellbeing at the government authority Public Health England, said the findings held a clear message for tobacco smokers.

"Switching to e-cigarettes can significantly reduce harm to smokers, with greatly reduced exposure to carcinogens and toxins," he said in a statement. "The findings also make clear that the benefit is only realised if people stop smoking completely and make a total switch.

"The best thing a smoker can do, for themselves and those around them, is to quit now, completely and forever."