Smoking increases the risk of breast cancer in older women by almost a fifth, a study has found.
The discovery adds to a growing weight of evidence linking exposure to tobacco smoke with the disease.
US scientists who tracked the progress of around 186,000 women aged 50 to 71 found that those who smoked were 19 per cent more likely to develop breast cancer than never-smokers.
Women who once smoked but then kicked the habit were still 7 per cent more at risk.
The results held true even after accounting for alcohol consumption, which is a major breast cancer risk factor that is more common among smokers.
In the last few years new findings have led to a U-turn in expert opinion.
In 2009, the International Agency for Research on Cancer concluded there was “limited evidence” that smoking tobacco can trigger breast cancer.
Last year, researchers from the American Cancer Society reported results showing a 24 per cent higher rate of breast cancer among women who smoked.
The risk was much greater among those who started smoking young, either before they started menstruating or before having their first child.
The new study, published in the British Journal of Cancer, covered a period of around 10 years.
During this time, 7500 of the women were diagnosed with breast cancer.
Dr Sarah Nyante, from the US National Cancer Institute in Bethesda, Maryland, said: “Our study adds to a growing body of evidence that suggests an association between cigarette smoking and increased breast cancer risk.
“Previous studies have investigated this relationship, but questions remained regarding the extent to which other breast cancer risk factors, such as alcohol intake, might influence the results.
“More work is now needed to understand the mechanisms behind the link between smoking and breast cancer in post-menopausal women.”
While other lifestyle factors such as being overweight, drinking alcohol and lack of exercise are all known to increase breast cancer risk in post-menopausal women, the role of smoking has been more difficult to unravel.
Smoking is the biggest preventable cause of cancer worldwide.
The habit is linked not only to lung cancer but cancers of the larynx, oesophagus, oral cavity, pharynx, bladder, pancreas, kidney, liver, stomach and bowel.
In the past 50 years, it is estimated that 6.5 million people in the UK have died from tobacco-related diseases.
Dr Julie Sharp, head of health information at Cancer Research UK – which owns the British Journal of Cancer, said: “Evidence remains inconsistent as to whether smoking causes breast cancer after as well as before the menopause, but this study suggests it may increase a post-menopausal woman’s risk of breast cancer if she smokes or has smoked in the past.
“Quitting is not easy but, given that smokers lose an average of 10 years of life compared to non-smokers, the benefits are huge.”