Cancer rates worldwide could increase by 50 per cent by 2020, reaching 15 million new cases a year.

The World Health Organisation's World Cancer Report says that if no preventive measures are taken increases of this order are inevitable because they stem from habits - such as smoking - that are already established. However, much can be done to slow the increase, Bernard Stewart, an Australian cancer specialist and co-author, said.

"Tobacco is the major preventable cause of cancer," he said. "But in developed countries diet, alcohol, a lack of exercise and the fact that we still don't eat five portions of fruit and vegetables a day are contributory factors.

Professor Stewart cited the cases of Finland and California. In Finland a programme of tobacco control, public education and dietary advice had seen lung cancer rates tumbling.

In California, the most anti-smoking state in America, lung cancer rates in women were falling. Everywhere else in the US they were rising.