A new study has said that older women who are dangerously overweight are at a far higher risk even than those who drink heavily or smoke to develop breast cancer.
According to data released by Cancer Research UK, one in eight women in the UK develop breast cancer in their lifetime, and the majority of these tumours are 'hormone sensitive' meaning their growth is fuelled by hormones.
Studies show that post-menopausal women with high levels of oestrogen and testosterone have between two and three times the risk of breast cancer than women with the lowest levels.
The Oxford University team studied the health records of nearly 6,300 post-menopausal women, looking for factors that might explain why some developed hormone sensitive breast cancer when others did not.
A woman's weight had the greatest bearing on a woman's sex hormone levels, shortly followed by smoking and alcohol consumption.
Women who are overweight or obese had high levels of hormones like oestrogen and progesterone.
So too did women who drank more than two and a half units of alcohol a day or smoked more than 15 cigarettes daily.
Experts say women should be made aware of these modifiable risk factors.
"This is an important study as it helps to show how alcohol and weight can influence hormone levels. Understanding their role in breast cancer is vital and this analysis sheds light on how they could affect breast cancer risk," the BBC quoted Dr Julie Sharp, of Cancer Research UK, as saying.
"We know that the risk of the disease can be affected by family history and getting older, but there are also things women can do help reduce the risk of the disease. Maintaining a healthy body weight and reducing alcohol consumption are key to reducing breast cancer risk," she added.
The findings appear in the British Journal of Cancer.