At least half, and perhaps as many as two-thirds, of people who begin smoking in their youth are eventually killed by the habit, according to a mammoth 50-year medical study by British researchers released Tuesday.

The report in the British Medical Journal (search) is the second part of a study that began in 1951. The publication's first report on the study in June 1954 was considered a landmark, confirming the link between smoking and lung cancer.

The second part of the study tracked the same participants for the past 50 years and highlights the findings that kicking the habit can add years to the lives of former smokers.

Researchers Richard Doll and Bradford Hill — the same two scientists who published the 1954 report — said that, on average, cigarette smokers die about 10 years younger than nonsmokers.

The main tobacco-related causes of death included lung cancer; heart disease; cancers of the mouth, pharynx, larynx and esophagus; and other respiratory diseases.

Those who stopped at the age of 60 gained three years of life, those who quit at 50 gained six years and those who stopped smoking at 40 gained nine years of life expectancy, the report said. For those who kick the habit at 30 the increased risk was avoided almost totally, the researchers said.

"Over the past few decades prevention and better treatment of disease have halved nonsmoker death rates in the elderly in Britain," said Doll, emeritus professor of medicine at the Clinical Trial Service Unit (search) at Oxford University, who is now in his 90s.

"But these improvements have been completely nullified by the rapidly increasing hazards of tobacco for those who continue to smoke cigarettes," he added.

Sir Richard Peto, professor of medical statistics and epidemiology at Oxford, said that tobacco had caused six million deaths in Britain in the last 50 years, but worldwide would soon be killing six million a year.

"The standard message on smoking is about half of all smokers get killed by it and stopping works," Peto said.

Britain's Tobacco Manufacturers' Association (search) said it had no immediate comment on the study.

The study, funded by the Medical Research Council, Cancer Research U.K. and the British Heart Foundation, followed almost 35,000 male doctors born between 1900 and 1930.

In the group of men born around 1920, about two-thirds of those who continued to smoke were killed by tobacco, the study concluded.

The research said that the high risks for that group could be linked to conscription into the British Army from 1939 onwards. Soldiers were given access to low-cost cigarettes, a factor which could have established the addiction with an intense early exposure to smoking.