Men with numerous lovers have a lower risk of developing prostate cancer-because they have more sex, scientists say.

Lotharios who have had more than 20 partners have a 28 per cent reduced risk compared with men who have had only one sexual partner in their lifetime, they claim.

But it is the amount of sex they have rather than the number of partners that cuts the cancer risk.

Lead researcher Prof. Marie-Elise Parent, from the University of Montreal, said: ‘It is possible that having many female sexual partners results in a higher frequency of ejaculations, whose protective effect against prostate cancer has been previously observed in cohort studies.’

Men who are virgins are almost twice as likely to be diagnosed with prostate cancer as those who are sexually experienced, it was claimed.

One theory is that large numbers of ejaculation may reduce the concentration of cancer-causing substances in prostatic fluid, a constituent of semen.

But the same is not true for gay men-Where having more than 20 male partners doubles the risk of prostate cancer.

The danger of a less aggressive cancer type also rises five-fold, possibly to do with greater exposure to sexually transmitted infections or the forum of gay sex.

More than 3,200 men aged between 40 and 79 answered questions about their lifestyle and sex lives in the prostate cancer and environment study.

Prof. Parent added: ‘We were fortunate to have participants who were comfortable talking about their sexuality.’