Widespread dropping of the habit in Britain since then has halved the number of lung cancer deaths - the world's largest decrease - that otherwise would be occurring, said Oxford study leader Sir Richard Peto.

Worldwide, smoking deaths are increasing because people in other countries started smoking more recently and a lag exists between taking up smoking and dying from it.

His research found there already are 1 billion smokers and that, by 2030, another billion or so young adults will have started to smoke. If current smoking patterns continue, worldwide tobacco deaths will rise from 4 million a year now to about 10 million a year around 2030, an increase of about 100 million a decade, Peto estimated.