Semen can speed up the progression of cervical cancer and cancer of the uterus, according to research out today.
Women suffering either cancer are being urged to ask their partners to wear condoms during sex.
A team of scientists led by Dr Henry Jabbour at the Medical Research Council's Human Reproductive Sciences Unit found that tumours grew in the presence of the compound prostaglandin.
It occurs naturally in the cells of the female reproductive organs but is 1,000 times more highly concentrated in semen.
Dr Jabbour and his team examined the effects of prostaglandins on the growth of tumour tissue.
He said: "What we have demonstrated is that the levels of the receptors for prostaglandins are elevated in these cancers.
"That elevated level of the receptor drives the growth of the tumour.
"What is unique about reproductive cancers is that, even if we blocked the synthesis of prostaglandins in the reproductive organs, it would not have any effect on the cancer if the woman was sexually active."
Therefore, he said, it was shown that it was the prostaglandins in the semen that were driving the tumour growth forward.
He added: "Sexually active women who are at risk of cervical or uterine cancer should encourage their partners to wear a condom to prevent increased exposure to the prostaglandins that may make their condition worse.
"This research also highlights the potential for a new therapeutic approach that will tackle both possible sources of prostaglandin, those produced naturally by women and those introduced to the body by sperm."
Cervical cancer, the first signs of which can be picked up during smear tests, is usually triggered by the human papilloma virus (HPV).
Professor John Toy, medical director at Cancer Research UK, said: "This is an interesting piece of laboratory research but it has little relevance to women already diagnosed with cervical cancer in the UK because they will already be receiving appropriate anti-cancer treatment.
"The likelihood of any unprotected sex affecting the successful outcome of their treatment is considered slight.
"The most important thing that women can do at this time to prevent cervical cancer from developing is to go for regular cervical smear tests."
The MRC findings have been published in the Journal of Endocrinology and Human Reproduction.