Smoking cigarettes cuts an average of 10 years off a person's life, a landmark study suggests.
About 12m British adults smoke
But it also shows that quitting at any age reduces the risks of dying from smoking-related diseases.
The findings, published in the British Medical Journal, are the culmination of a 50-year study involving 34,439 men.
The study, which began in 1951, was the first to confirm the link between smoking and lung cancer exactly 50 years ago.
Fifty year study
All of those involved in the study were born between 1900 and 1930 and all worked as doctors.
They were each asked about their smoking habits at the start of the study in 1951.
Researchers contacted them periodically over the next 50 years to see if those habits had changed.
They also gathered information on those who died during the period.
They have now analysed that data.
They found that men who have never smoked lived on average 10 years longer than those who smoked for most of their lives.
Men who smoked were at least twice as likely to die before the age of 70 as non-smokers.
They were up to three times more likely to die before they were 90 compared to those who never took up the habit.
The average age of these men when they started smoking was 18. On average, they said they smoked around 18 cigarettes a day.
It was already known that about half of all persistent cigarette smokers are killed by their habit, a quarter while still in middle age (35-69 years). The biggest killers are cancer, heart disease and stroke.
But the study also revealed that giving up cigarettes at any age has major health benefits.
It found that men who had stopped smoking by the time they were 30 lived as long as those who never smoked.
Those who quit at 40, lived just one year less than those who had never smoked.
Those who stopped smoking at 50 added six years onto their lives, while those who kicked the habit at 60 added an extra three years to their life.
Sir Richard Doll, who has been involved in the study since the start, said smoking was still a major killer in the UK.
"Over the past few decades prevention and better treatment of disease have halved non-smoker death rates in the elderly in Britain.
"But these improvements have been completely nullified by the rapidly increasing hazards of tobacco for those who continue to smoke cigarettes."
Sir Richard Peto, professor of medical statistics and epidemiology at Oxford University has collaborated on the study for 30 years. He said smoking was responsible for millions of death.
"Partly because of earlier results from this 50-year-long study many people in Britain gave up smoking, and this country now has the best decrease in tobacco deaths in the world.
"But, in many countries tobacco deaths are still going up. In Britain, tobacco has caused six million deaths over the last 50 years. But, worldwide, tobacco will soon be causing six million deaths each year."
Professor Sir Charles George, medical director of the British Heart Foundation, welcomed the study.
"It provides a clear demonstration of the harmful effects of persistent cigarette smoking, which on average shortens life by around 10 years.
"Both for heart disease and for cancer the benefits of stopping smoking are clear cut and the earlier this occurs, the better."
Professor Alex Markham of Cancer Research UK said: "Tobacco is responsible for a third of all UK cancer deaths. Since the study began in 1951, tobacco has killed around 100 million people globally."
Professor Colin Blakemore, chief executive of the Medical Research Council, said: "Fifty years ago, the findings of this unique study had a major impact on our understanding of the links between smoking and disease.
"These new findings complete the picture on smoking-related deaths."
Simon Clark, director of the smokers' rights group Forest said everybody knew there were risks associated with smoking.
"However tobacco is still a legal product which brings a lot of pleasure to many people, including a significant number of doctors.
"Government has a role to play educating people about the health risks but in a mature society people should have the freedom to make an informed choice without being patronised or forced to give up."