Older women who are overweight may be more likely to get breast cancer, according to a study which found that high levels of sex hormones which are more common in those who are overweight can double the risk.

The paper, published by scientists from Cancer Research UK's epidemiology unit in Oxford, pooled data from nine separate studies in the UK, US, Italy and Japan which included 765 post-menopausal healthy women and 663 with breast cancer.

They found that those who had high levels of testosterone or free estradiol - an active form of oestrogen - in their blood had more than twice the risk of breast cancer of those who did not. Women with raised levels of a molecule called sex hormone binding globulin, which reduces the activity of sex hormones, were at lower risk of breast cancer. Women who are obese tend to have low levels of this molecule in their blood.

"We've known for some time that reproductive and hormonal factors were involved in the development of breast cancer. Until now, we haven't had strong evidence for a direct link between the blood levels of natural hormones and risk of the disease. Our study confirms that high levels of sex hormones can raise risk," said Tim Key, leader of the team which published the paper in the Journal of the National Cancer Institute yesterday.

"In the future, our research may allow testing to predict a woman's risk of breast cancer and could provide leads for improved drugs to help prevent the disease. But meanwhile it's important that women try to maintain a healthy weight, since this will reduce their blood oestrogen levels and breast cancer risk."

"Obesity is responsible for 7% of the UK's cancer deaths," said Sir Paul Nurse, director general of Cancer Research UK. "In the future, it may be possible to reduce the risk of cancer in women with high levels of sex hormones."