It follows tests on rats which suggest a chemical in soya can damage the male reproductive organs.

There is no evidence to suggest it has a similar effect on humans - there have been no such reports from Asia, where soya is a major part of the diet.

These are serious questions that need answering   Chris Kirk, University of Birmingham

However, researchers in the United States said its effects on rats were so severe that tests were needed to ensure it was safe for pregnant women to eat.

Scientists at the Johns Hopkins School of Public Health fed pregnant rats a diet laced with genistein - a chemical found in soya.

Large amounts of genistein are found in some baby formula milks and in some supplements taken by women as an alternative to hormone replacement therapy.

High levels

According to a report in New Scientist magazine, the scientists found that male rats exposed to high levels of genistein in the womb grew up to have larger prostate glands and smaller testes.

The rats had normal sperm counts and would behave as if they wanted to mate when placed with females. However, they were unable to ejaculate.

The effects were just as severe in males that did not eat genistein after weaning as it was in those that continued eating it.

They suggested that this indicated that exposure in the womb and during breast feeding has the biggest impact.

The scientists also found that male rats exposed to genistein had a slightly large thymus gland, an organ that produces immune cells.

This contradicts previous studies which suggested it has the opposite effect on the thymus gland.

To add to the confusion, the scientists also found that moderate levels of genistein had an even bigger effect on males rats than large doses.

The researchers said their findings had caused them concern.

Sabra Klein, one of those involved in the study, said: "The urologists on this project are actually advising pregnant women to avoid soya."

Chris Kirk, who is based at the University of Birmingham, said the findings highlighted the need for further research.

"These are serious questions that need answering," he told New Scientist.