Professor Richard Peto, professor of medical statistics and epidemiology at Oxford University, told the British Association of Science conference that many thousands of middle aged Britons were alive today because they had given up smoking.

Professor Peto said that, in 1965, 42% of men died before the age of 70 and nearly half of these deaths were due to smoking.

By 1995 the death rate dropped to 28% and only a third were attributed to tobacco.

Professor Peto said: "The decrease in mortality is being driven by a decrease in tobacco."

Class differences

Smoking was also the cause of most of the differences in death rates between the rich and poor, Professor Peto added, because the people from lower social classes and on smaller incomes are more likely to take up the habit.

He told the conference that the best way to reduce death rates worldwide was to target tobacco - which along with the HIV virus was the biggest killer. Approximately 100 million people will die from smoking worldwide over the next 20 years.

Professor Peto said: "You can save far more lives by a moderate reduction in the big causes of death than by a large reduction in smaller causes. Tobacco is still the biggest killer we've got."

"Half of all smokers are killed by tobacco, but stopping works amazingly well; even in middle age smokers who stop avoid most of their risk of death from tobacco and stopping before middle age avoids almost all the risk."

Smoking is linked to premature death

Clive Bates, director of Action on Smoking and Health, welcomed Professor Peto's comments as yet more evidence that smoking was harmful and linked to premature death.

Mr Bates said: "It is good news. It shows that if you do give up smoking you will get a health benefit and that smokers are not irretrievably doomed."

Child death rates falling

Professor Peto said infant deaths had decreased at amazing rates in the last 100 years, with only about 1% of children worldwide dying before their fifth birthday.