Men whose index fingers are longer than their ring fingers are significantly less likely to develop the disease.
The reason is thought to be hormonal influences in the womb. Less exposure to the sex hormone testosterone results in a longer index finger.
Researchers made the discovery after comparing 1,500 prostate cancer patients and about 3,000 healthy men. More than half the men in the study had a shorter index finger. They developed prostate cancer at about the same rate as men whose index and ring fingers were the same length. But in those whose index fingers were longer, the risk was reduced by 33 per cent.
Professor Ros Eeles, of the Institute of Cancer Research in Surrey, said the results could provide a simple cancer risk test.