Mobile phones have been identified as a cause of reduced sperm production in men.

Microwaves emitted by the phones reduce the number, mobility and quality of sperm by almost half in the heaviest users, to the point where some men may become infertile, scientists say.

This could have devastating consequences for fertility rates around the world.

Almost a billion people around the world use mobile phones, and the number is growing.

Even a small effect on fertility could result in millions of men being rendered childless.

Concern about the health effects of mobile phones has been rising for 10 years, but very little hard evidence of the dangers has been presented.

Scientists from the Reproductive Research Centre at the Cleveland Clinic Foundation in Ohio, tested the sperm of 364 men being investigated for infertility.

They found that the heaviest users of mobile phones - more than four hours a day - had the lowest sperm counts at 50 million a millilitre and the least healthy sperm, judged by its mobility and the proportion of abnormal sperm.

Sperm counts were highest - 86 million a millilitre - and the sperm healthiest among those men who did not use mobile phones.

All men produce a high proportion of sperm that are abnormal, but in the heaviest mobile users the "normal" sperm fell to 18 per cent compared with 40 per cent in those who never used mobiles.

The study was carried out in Bombay, India, where mobile phones have not yet penetrated all social groups.

Professor Ashok Agarwal, director of the research centre, who led the study, said: "On all four parameters - sperm count, mobility, viability and morphology - there were significant differences between the groups.

"The greater the use of cell phones, the greater the decrease in these parameters. That was very clear and very significant.

"People use mobile phones without thinking what the consequences may be. It is like using a toothbrush - but mobiles could be having a devastating effect on fertility. It still has to be proved, but it could have a huge impact because mobiles are so much part of our lives."

Among the heaviest users in the study, with an average sperm count of 50 million a millilitre, some had individual sperm counts of less than 20 million a millilitre.

This is below the threshold set by the World Health Organisation which defines infertility, Professor Agarwal said.

The finding, presented to the American Society for Reproductive Medicine meeting in New Orleans this week, will spark renewed concern about the safety of mobile phones which have been blamed for a wide range of ill effects, from headaches to cancer.