A Mediterranean diet can reduce the risk of a deadly form of breast cancer by 40 per cent, a large study has found.

Scientists monitored more than 62,000 women over a period of 20 years to see how their breast cancer risk was affected by what they ate.

Those who adhered most closely to a Mediterranean diet rich in plant protein, fish and olive oil were 40 per cent less likely to develop ER-negative breast cancer than women who adopted the diet the least.

This form of the disease, which is not stimulated by the sex hormone oestrogen, is often harder to treat than hormone-sensitive cancer and more likely to prove fatal.

A typical Mediterranean diet includes high intakes of plant-based proteins such as nuts, lentils and beans, wholegrains, fish and “healthy” monounsaturated fats such as olive oil. Consumption of refined grain foods such as white bread or white rice, red meat and sweets is kept to a minimum.

Piet van den Brandt, a professor from Maastricht University in the Netherlands, who led the study published in the International Journal of Cancer, said: “Our research can help to shine a light on how dietary patterns can affect our cancer risk. We found a strong link between the Mediterranean diet and reduced oestrogen-receptor negative breast cancer risk among post-menopausal women, even in a non-Mediterranean population.

“This type of breast cancer usually has a worse prognosis than other types of breast cancer”.

Alcohol, which normally forms part of a traditional Mediterranean diet, was excluded from the study because of its known links to breast cancer.

Panagiota Mitrou, director of research funding at the charity World Cancer Research Fund, which funded the research, said: “This important study showed that following a dietary pattern like the Mediterranean diet, could help reduce breast cancer risk – particularly the subtype with a poorer prognosis. We would welcome further research that helps us better understand the risk factors for the different breast cancer subtypes.”

Following a Mediterranean diet only had a weak non-significant effect on the risk of hormone-sensitive ER-positive breast cancer, the study found.

“We know how devastating a diagnosis is and this study adds to evidence that a healthy diet, full of ‘good’ low-saturated fats, plays a part in lowering risk of the disease,” said Emma Pennery, clinical director of the charity Breast Cancer Care. “However, it’s important to remember while lifestyle choices like eating a well-balanced diet and taking regular exercise can help reduce the risk of cancer, they don’t guarantee prevention.”