Having too many sexual partners may increase a man's risk of developing prostate cancer, scientific evidence in Britain has revealed.

An article appearing in the Daily Mail newspaper last summer indicated that a man's sexual past can come back to haunt him years later. Research shows that having unprotected sex with multiple partners can double the chances of developing prostate cancer, which is predicted to become the most common form of the disease in the UK in 20 years.

Experts say the findings suggest that a sexually transmitted infection may play a part in the development of the disease. Professor Karin Rosenblatt of the University of Illinois led the research and warned that it `suggests there is a direct positive relationship between the number of lifetime female sexual partners and the risk of prostate cancer in middle-aged men'. Professor Rosenblatt added that sexual behaviour and associated exposure to sexually transmitted agents appeared to enhance the risk. Her research was based on a comparison of the lifestyles of 750 men who had been diagnosed with prostate cancer between 1993 and 1996 and a similar number who had remained healthy.

The men, aged between 40 and 64, were asked how many sexual partners they had had in their lifetimes. Those who said 30 or more were found to have more than twice the risk of prostate cancer. Those with just one sexual partner throughout their lives had the lowest risk.

Professor Jonathan Waxman, professor of Oncology at the Hammersmith Hospital in West London and chairman of the Prostate Cancer Charity, found the findings interesting.

"There's been a long suspicion of this but it has never really been confirmed until now," said Professor Waxman.

"It does point the way to there being an infectious, viral agent involved in the passage of the disease. But we are really at a loss to understand this from a scientific viewpoint.

"There's no scientific evidence that an infective agent is causing prostate cancer. But this is, I suppose, another reason for men not to lead promiscuous lifestyles."

The scientists stressed there was no relationship, however, between the number of times a man had sex in his lifetime and the cancer risk. In fact, doctors believe that regular sex can actually be important in the prevention of prostate cancer in later life because it cleanses the prostate gland.

It was only the number of different partners that raised the chances of a tumour developing.