Being overweight contributes more to the most common form of breast cancer than alcohol or smoking, a study suggests.

Excess weight had the strongest influence on levels of the female sex hormone oestrogen, which are known to fuel around three-quarters of breast tumours.

Scientists analysed data on almost 6,300 post-menopausal women from 13 different studies looking for the chief factors affecting sex hormones.

Body Mass Index scores, which measure weight in relation to height, had the biggest impact on sex hormones, especially oestrogen.

It was already known that post-menopausal, obese women have a higher risk of breast cancer.

Women who drank two and a half units of alcohol a day, equivalent to a medium-sized glass of wine, also had higher levels of sex hormones, as did those who smoked 15 cigarettes a day. However, excess weight was the most important factor.

Co-author Dr Gillian Reeves, from Oxford University, said: “Our study shows that changes in hormone levels might explain the association of established risk factors such as obesity with breast cancer risk.

“Other studies have found that weight and alcohol can affect hormone levels, and this research confirms and adds to these findings and provides more information about how breast cancer develops.”

Julie Sharp, senior science information manager at Cancer Research UK, said: “There’s already strong evidence that drinking alcohol and being overweight can increase the risk of breast cancer, but it’s important to understand why these links exist.

“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.

“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.”