Drink a day increases breast cancer risk
However smoking, which causes 15 other types of cancer, does not contribute to breast cancer, according to the world's largest study of women's tobacco and alcohol habits.
The study by Cancer Research UK suggests alcohol accounts for 4% of breast cancers in the developed world and around 2,000 cases each year in the UK.
And the charity warned that if women's alcohol consumption continues to increase, this figure is likely to rise.
However the researchers also pointed out that women who drink are at a lower risk of heart disease and stroke than non-drinkers.
Dr Gillian Reeves, one of the report's authors, 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."
The researchers combined the results of more than 50 studies and included data on around 150,000 women from across the world.
Because of the scale of the study, published today in the British Journal of Cancer, they were able to separate out tobacco and alcohol use.
More than 23,000 of these women did not drink and among them there was no significant difference between the rates of breast cancer in smokers and non smokers.
However Sir Richard Doll, a co-author of the study, said smoking is known to cause 15 other types of cancer.
"While breast cancer is the most common form of cancer in women, survival rates are relatively good," he said.
"A woman is more likely to die of lung cancer because it is notoriously difficult to treat."
The average alcohol intake for women in the UK has increased from around seven grammes to eight grammes (or one unit) per day in the last decade but among younger women the increase has been greater.
It is not clear from the study whether the increased relative risk of breast cancer for women who drink comes from their current consumption of alcohol or if is based on long term or past drinking habits.
The researchers said they did not know why alcohol increased the risk of breast cancer but said it might be that it affects oestrogen levels.
The overall risk of breast cancer means that by age 80 there will be 8.8 cases per 100 women.
This increases to 9.4 cases for those who have one alcoholic drink daily, and goes up to 13.3 cases for those who have six drinks each day.
Professor Valerie Beral, of Cancer Research UK's Cancer Epidemiology Unit at Radcliffe Infirmary in Oxford and a co-author, said: "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 rate of breast cancer in the future.
"Alcohol is a small part of the breast cancer story but it is something that is easy for women to change."
Sir Paul Nurse, chief executive of Cancer Research UK said: "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