“Our findings suggest that the greater the soya food intake is, the lower the sperm concentration, compared with men who never consume soya food,” said lead researcher Jorge Chavarro, at Harvard school of public health in Boston.

The study, which appears in the journal Human Reproduction, found 41 million fewer sperm per millilitre of semen after just one portion every two days.

It is thought that soya compounds called isoflavones, which mimic the female sex hormone oestrogen, are behind the effect.

Earlier animal studies have linked a high consumption of isoflavones with infertility.

In the latest study on 99 men, who visited a fertility clinic between 2000 and 2006, US researchers led by Chavarro at Massachusetts General hospital found that men who consumed at least half a portion of soya food a day had the lowest sperm counts.

The “normal” sperm concentration for a man is between 80 and 120 million per millilitre, and the average of men who ate on average a portion of soy-based food every other day was 41 million fewer, the study said.

The apparent fall in sperm count is unlikely to make healthy men infertile, but some experts said it could have a significant impact on those already with lower than average sperm counts.

Soya-based products are now found in two-thirds of manufactured food including biscuits, sweets, pasta and bread, according to the Institute of Food Research in Norwich.