Switching to e-cigarettes could save millions of smokers' lives, a conference on the rapidly expanding use of the devices heard Tuesday, though some delegates warned more research on the health effects is needed.

The merits of e-cigarettes were thrashed out at a one-day gathering of some 250 scientists, experts, policymakers and industry figures at the Royal Society in London.

The use of electronic cigarettes -- battery-powered devices that simulate smoking by heating and vaporising a liquid solution containing nicotine -- has grown rapidly, with tobacco manufacturers jumping on the trend.

Sales have doubled annually for the last four years and there are an estimated seven million users across Europe.

European lawmakers in October threw out a bid to curb sales of e-cigarettes by classifying them as medicinal products.

Delegates in London debated how the market had moved faster than science or the law.

Many delegates merrily "vaped" away throughout the conference sessions, including one man puffing away on an e-pipe and another inhaling an e-cigar that lit up blue.

"Cigarettes are killing 5.4 million people per year in the world," said Robert West, a health psychology professor and the director of tobacco studies at Cancer Research UK.

He told delegates switching to e-cigarettes could save millions of lives, but the debate was about "whether that goal can be realised and how best to do it".

The professor said almost a third of attempts to quit smoking involved e-cigarettes.

E-cigarettes 'running ahead of science'

Though they are estimated to be between 95 and 99 percent safer than smoking cigarettes, some countries have banned them and attendees debated the thorny issue of whether regulation -- possibly under medicinal rules -- should be brought in.

They also debated their potential as a tool to quit smoking or whether they might become a gateway for those who have never smoked.

Deborah Arnott, chief executive of the Action on Smoking and Heath (ASH) pressure group, said e-cigarettes could be a leap forward for public health but warned that not enough was known about their effects -- and pointed out that the tobacco companies are snapping up the e-cigarette manufacturers.

"ASH thinks that e-cigarettes have significant potential. They are a lot less harmful than smoking. Clearly smokers find them attractive, primarily as a way of quitting and moving away from smoking, which they know will kill them," she told AFP.

"I think the jury's out and these products need regulating because there's a real concern that their safety and effectiveness is not guaranteed without regulation.

"The tobacco companies are moving in. For them it's potentially a 'Kodak moment' because if everyone moved to e-cigarettes, they'd lose their market so they've got to be in there. A lot of the bigger e-cigarette companies have already been bought up."

She warned: "If there are carcinogens in there, you won't see an immediate effect but 10, 15, 20 years down the line, people will be dying from that.

"The development of e-cigarettes is definitely running ahead of the science."

Jacques Le Houezec, a consultant in public health and tobacco dependence from France, confirmed the booming growth was being driven by smokers, not laboratory experts.

He told delegates that while e-cigarettes contained some harmful substances, the levels of toxicants were nine to 450 times lower than in cigarette smoke.

Matthew Moden, a director of English firm Liberty Flights, which makes e-liquid from pharmaceutical-grade nicotine, said the market had grown exponentially.

Since starting in 2009 as an online retailer with three workers, the company now employs 75 people and has 10 stores.

They retail a starter kit for 25 pounds ($40, 30 euros), while a refill costs 4.85 pounds for 10 millilitres. An average user might get through one to three millilitres per day, he said. The e-liquids come in a multitude of flavours, including menthol and forest fruits.

"Our customers always refer to this as the product that helped them to stop smoking," he told AFP.

"We welcome regulation. We need to be transparent with what we're selling, how safe it is and where it comes from.

"In 10, 12, 20 years' time, I think nicotine will be akin to caffeine in regard to how people consume it, how it's considered and the effect it has on the body."