A woman's risk of breast cancer increases by 6 per cent for every extra alcoholic drink consumed on a daily basis, according to the world's largest study of women's smoking and drinking behaviour.
The study, published in the British Journal of Cancer this week, also found that smoking, which causes a third of all cancers, does not contribute to breast cancer.
The new research from Cancer Research UK estimates that alcohol accounts for around 4 per cent of breast cancers in the developed world and around 2,000 cases each year in the UK alone. And if women's alcohol consumption continues to increase this figure is likely to rise.
In the past it has been extremely difficult for researchers to separate the effects of tobacco from the effects of alcohol because the more women drink the more they tend to smoke and vice versa. This is a major reason why previous work has yielded conflicting results, said the research agency.
But the sheer size of the new study allows the researchers to disentangle the two factors and make the most accurate estimates ever of the risks associated with smoking and drinking. Researchers combined results from more than 50 studies and included data on around 150,000 women from around the globe. Over 23,000 of these women did not drink, and looking at this group separately, the researchers could see no significant difference between rates of breast cancer in smokers and non-smokers.
Sir Richard Doll, a co-author of the study, said: "There has been a great deal of research on whether smoking or alcohol contribute to breast cancer but until now results have been confused.
"For the first time we have undertaken a study large enough and detailed enough to look at the separate effects of tobacco and alcohol reliably. When we did this we found that drinking, but not smoking, increases the risk of breast cancer."He added: "While breast cancer is the most common form of cancer in women, survival rates are relatively good. A woman is more likely to die of lung cancer because it is notoriously difficult to treat and lung cancer is dependent on smoking but not drinking."
Co-author Professor Valerie Beral, of Cancer Research UK's Cancer Epidemiology Unit at the Radcliffe Infirmary in Oxford, said: "This research tells us there is a definite link between alcohol and breast cancer and the evidence suggests that the more a woman drinks the greater her risk.
"The impact of drinking on breast cancer is small compared to childbearing factors, but women are drinking more now than they used to and if this pattern continues it is bound to have an impact on the rates of breast cancer in the future."
The average alcohol intake for UK women has increased from about 7 grams to 8 grams per day in the last decade, but for young women the increase has been greater. Among women aged between 16 and 24, the proportion drinking more than three drinks per day has doubled from 9 per cent to 18 per cent.
While women who drink are at a higher risk of a number of diseases, including cancers of the throat and liver, they are at a lower risk of heart disease and stroke than non-drinkers.
Dr Gillian Reeves, who also co-authored the report, said: "The balance between the harmful effects of alcohol on breast cancer and its beneficial effects on heart disease depend on a woman's age.
"Before about 60, breast cancer is a more important cause of death than heart disease. After the age of 65 or so, when the risk of heart disease becomes much greater than the risk of breast cancer, the benefits of moderate drinking are more apparent."
Sir Paul Nurse, Cancer Research UK's chief executive, said: "Large studies of this kind are very important for dissecting the complex causes of cancer. This research doesn't alter our advice on smoking, because we already know that it's dangerous, but it does reinforce our advice that excessive drinking can also be hazardous.
"It seems that women's attitudes to alcohol are changing and this can only have a negative impact on their health. It's important that we get the message out to young women that drinking too much is dangerous."