Laboratory tests suggest the naturally occurring chemical destroys the mechanism that allows sperm to dock with women's eggs, she said at the European Society of Human Reproduction and Embryology in Copenhagen.

Her team is about to test the theory by mating mice on a soya diet.

"It might be practical, if you are in the habit of eating lots of soya-based products, to restrict your diet for a short time over your window of ovulation," Fraser said.

The researcher, of King's College, London, added that sperm could "hang around" for four days in women's organs.

Soya, present in products such as bread, milk, margarine, ready meals and sauce, is often lauded for preventing damage to cells, and protecting them against heart disease and some cancers.

However, even the Vegetarian Society went along with Fraser's advice. "For anyone struggling to become pregnant, avoiding soya products for a few days a month is worth a try if there is even a slim chance that it will help fertility." It recommended alternatives for vegetarians and vegans, including Quorn meat substitutes, oat or rice milk and pulses.

Fraser said human sperm had proven 10 to 100 times more sensitive than mouse sperm to the action of genistein. "Human sperm are responding to very low concentrations -- well within the amounts that have been measured in people's bloods."

Fraser said research indicated "important warning signs" but cautioned that little was known about how sperm worked in human bodies rather than laboratories.

Professor Richard Sharpe of the Human Reproductive Science Unit in Edinburgh,was sceptical of the theory. "Oriental societies that traditionally eat a soy-rich diet show no signs of reduced fertility of which I am aware; (and) effects on sperm in the laboratory are not necessarily directly related to what might happen in real life."