A study of more than 6,000 Swedish men over 30 years found that the eating of fish affected their chances of suffering the disease. Men who ate no fish had a two to three times higher risk of prostate cancer than those who ate moderate or high amounts.

Oily fish contain essential fatty acids which have previously been shown to inhibit the growth of prostate cancer cells. A key fatty acid is omega-3, which is often found in fish oil supplements.

The study published in the June 2000 issues of the Lancet medical journal, monitored the men between 1967 and 1997. During that time there were 466 diagnoses of prostate cancer, of which 340 were fatal.

Currently one in 13 men in Britain gets prostate cancer. The disease affects about 20,000 men each year, killing half.

Dr John Toy, medical director at Imperial Cancer Research Fund, said: "This study has surveyed an unusually large number of men and certainly adds to our knowledge. It also provides encouragement that eating fish is a practical thing men can do to help lower their own risk of prostate cancer.

"It is vital that men and women eat a healthy, balanced diet as it can help reduce the risk of cancer and other serious diseases. Imperial Cancer estimates that about 35 per cent of all cancers may be preventable by changing our diet."