A billion people will die from tobacco-related diseases such as cancer this century unless more are encouraged to quit, a UK expert warns.
Experts say more people must be helped to quit
In the last century the death toll was about 100m, including 7m in Britain.
Professor Sir Richard Peto told a cancer conference in Birmingham developing countries were likely to be hit hardest.
Many nations are cutting smoking, but rates are increasing in countries such as India and China.
Smoking currently kills about five million adults a year globally.
Each year, about 30m people take up smoking around the world, Professor Peto said.
He added: "If more than 20m of these continue to smoke and half are killed by their habit, then we are going to have more than 10m 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," warned the Oxford University Professor.
That is equivalent to about a sixth of the world's current population.
'Long way to go'
Although it is a global problem, some countries will be hit particularly hard, experts predict.
In China, a third of young men are dying from smoking-related diseases already.
Hungary is now the country with the worst tobacco death rate in the world.
Professor Peter Boyle, Director of the International Agency for Research on Cancer (IARC) from Lyon, France, said: "Part of the problem is where we are going to see the big impact, with population growth and population ageing over the next 20 to 50 years.
He said that to a certain extent, these countries were the least able to cope with the extra disease burden of smoking on top of existing disease issues.
"The impact of tobacco is going to be absolutely enormous unless we do something about it now."
Professors Boyle said more effort was needed to help smokers quit, to restrict tobacco advertising and impose smoking bans in countries. He said the experience of countries that have focused on such measures showed they could work.
"I think the message is beginning to get across but there is a long way to go," he said.
He said it was now essential to "get the facts turned into policy."
But Simon Clark, of the smokers' rights group Forest, said: "This seems like a suspiciously round figure.
"Such ludicrous estimates and calculations are so over the top that people are switching off, and the serious message underlying this type of estimate are being lost because a lot of people aren't listening."