Women who smoke into middle-age have three times the death rate of non-smokers and risk dying at least 10 years early, according to a definitive study of the effects of tobacco in more than a million women in the UK.

The good news, according to the study by a team of Oxford University researchers led by Sir Richard Peto, is that giving up cigarettes before the age of 40 reduces a woman's risk of smoking-related death by 90%. Quitting by 30 reduces it by 97%.

The study, published by the Lancet a day before the 100th anniversary of the birth of Sir Richard Doll, who first established the link between smoking and lung cancer, shows conclusively for the first time that the disastrous effects of smoking for men are no different for women.

Because men historically began to smoke in large numbers earlier than women – most of the first generation of women with a lifelong smoking habit were born in the 1940s – the life-shortening impact of cigarettes on them has not until recently been fully measurable.

"If women smoke like men, they die like men," said Peto. "But whether they are men or women, smokers who stop before reaching middle-age will on average gain about an extra 10 years of life."

The data comes from the Million Women Study, which recruited 1.3 million women between 1996 and 2001 when they went for breast cancer screening, aged between 50 and 65. The study, unique because of its size, has compiled a vast amount of information on women's health and led, among other things, to important findings on risk factors for breast cancer.

The women filled in questionnaires on their lifestyle, medical history and social background at the time of recruitment and again three years later. They have been followed up for an average of 12 years. At the start of the study, 20% were smokers and 28% former smokers.

Persistent smokers were nearly three times as likely to die over the study period as non-smokers. The results show that two-thirds of all deaths of smokers in their 50s, 60s and 70s are caused by smoking – not only through cancer but also other smoking-related illnesses such as heart disease and stroke.

The more women in the study smoked, the greater their risk of premature death. Even those who considered themselves social smokers, having between one and nine cigarettes a day, had double the death rate of non-smokers. The increased risks remained, even after adjusting for the fact that smokers were more likely to live in deprived areas, drink more than 14 units of alcohol a week and avoid strenuous exercise.

The age at which women started smoking was important. Picking up the habit at a young age increased the length of time for which they smoked and their risk of an early death. Although fewer women were now starting to smoke, those who are could be doing greater damage to their health. "In those days smokers were starting in late teens and now it is in their early teens," said Peto.

Although stopping well before the age of 40 will substantially reduce women's risk of dying early, "this does not mean … that it is safe to smoke until age 40 years and then stop, for women who do so have throughout the next few decades a mortality rate 1.2 times that of never-smokers," says the paper. "This is a substantial excess risk, causing one in six of the deaths among these ex-smokers."

Peto said the Million Women Study had allowed unprecedented insights into women's health prospects and could only have been carried out in the UK.

"It is completely brilliant because the NHS health records are so brilliant. You couldn't do it in any other country in Europe. We can send details of the women to the NHS central register and we get told what everybody died of. Every time they go to hospital, we know about it."

• This article was amended on 27 October 2012. The original article and headline said that 'Women who quit smoking before 35 cut risk of tobacco-related death by 97%." This has now been corrected to "before 30".