One sixth of the world's population stand to die from cancer and other tobacco-related illnesses, with the highest death toll in developing countries, Professor Sir Richard Peto told a cancer conference in Birmingham yesterday.
While the British are facing a possible smoking ban in public places - following in the wake of Ireland and New York - 30 million people take up the habit worldwide each year.
Sir Richard, professor of medical statistics and epidemiology at the University of Oxford, said: "If more than 20 million of these continue to smoke and half are killed by their habit, then we are going to have more than 10 million tobacco-related deaths a year. So in the present century, if we keep on smoking the way we are we will have about 1,000 million deaths."
In the last century the death toll from smoking related diseases was 100 million, including seven million in Britain. Smoking currently kills about five million adults a year globally.
Professor Peter Boyle, director of the International Agency for Research on Cancer, warned that developing countries were least able to cope with the extra disease burden of smoking, on top of existing disease.
Simon Clark, of smokers' rights group Forest, said that the figures were a gross over-estimate. "Such ludicrous estimates and calculations are so over the top that people are switching off, and the messages underlying this type of estimate are being lost because people aren't listening."