A study of 50,000 post-menopausal women found eating just a quarter of a grapefruit daily raised the risk by up to 30%.

The fruit is thought to boost levels of oestrogen - the hormone associated with a higher risk of the disease, the British Journal of Cancer reported.

But the researchers and other experts said more research was still needed.

The women had to fill in questionnaires saying how often they ate grapefruit and how big their portions were.

Oestrogen important

The researchers, at the universities of South California and Hawaii, found that women who ate one quarter of a grapefruit or more every day had a higher risk of breast cancer than those who did not eat the fruit at all.

Previous studies have shown that a molecule called cytochrome P450 3A4 (CYP3A4) is involved in metabolising oestrogen hormones.

And grapefruit may boost blood oestrogen levels by inhibiting this molecule, allowing the hormones to build up.

The researchers found that in women who ate at least a quarter of a grapefruit daily, levels of oestrogen were higher.

They said: "It is well established that oestrogen is associated with breast cancer risk.