An aspirin a day may reduce a woman's risk of developing the most common kind of breast cancer, researchers have found.

The wonder drug has already been shown to combat pain, rheumatoid arthritis, heart disease, strokes and some cancers, notably those of the lung, bowel and pancreas.

Now for the first time aspirin has been shown to lower the risk of hormone-sensitive breast cancer.

In 75% of breast cancers, tumours are fuelled by the female hormone oestrogen.

A large study involving 127,000 women found that aspirin was linked to a small reduction in the risk of having this form of the disease.

The women, aged 51 to 72, were enrolled into the National Institutes of Health AARP Diet and Health Study, a major US investigation into links between diet, behaviour and cancer.

Aspirin belongs to a class of medicines known as non-steroidal anti-inflammatory drugs (NSAIDs). The researchers found that NSAID use generally had no influence on overall susceptibility to breast cancer. But daily doses of aspirin resulted in a 16% reduction in risk for oestrogen-positive breast cancers.

Writing in the journal Breast Cancer Research, the authors led by Dr Gretchen Gierach, from the National Cancer Institute, Rockville, Maryland, concluded: "Daily aspirin use... appeared to offer some protection for ER+ (oestrogen positive) breast cancer in this population..

"Our results provide support for further evaluating relationships in prospective studies with well-defined measures of NSAID use by NSAID type... and by ER status.'' Aspirin and other NSAIDs block cyclo-oxygenase (COX) enzymes, biological catalysts which may affect the growth of cancer.

However, unlike other NSAIDs, aspirin's impact on COX enzymes is irreversible.