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

The use of electronic cigarettes - pen-sized battery-powered devices that simulate smoking by heating and vaporising a liquid solution containing nicotine - has grown rapidly. Sales have doubled annually for the last four years and there are an estimated seven million users across Europe.

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

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

Dr. Jacques Le Houezec, a French consultant in public health and tobacco dependence, told delegates that while e-cigarettes contained some harmful substances, the levels of toxicants were nine to 450 times lower than in cigarette smoke.