Mobile phones may be causing widespread damage to sperm production in men with potentially devastating consequences for global fertility rates, a study suggests.

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

Almost a billion people around the world are using mobile phones and in some countries the number is growing at 20 to 30 per cent a year. Scientists from the Reproductive Research Centre at the Cleveland Clinic Foundation in Cleveland, Ohio, tested the sperm of 364 men who were being investigated for infertility with their partners.

They found that men who were the heaviest users of mobile phones - more than four hours a day - had the lowest sperm counts at 50 million per millilitre (ml) and the least healthy sperm. In contrast, sperm counts were highest (86 million per ml), and the sperm healthiest, among those men who reported that they did not use mobile phones.

All men produce a high proportion of sperm which is abnormal - including the fertile - but in the heaviest mobile users the "normal" sperm fell to 18 per cent compared with 40 per cent in non-users.

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

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

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

He added: "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."

Some of the heaviest users had individual sperm counts of less than 20 million per ml. 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 yesterday, will spark renewed concern about the safety of mobile phones.

However, Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: "This is a good quality study but I don't think it tackles the issue. If you're using your phone for four hours a day, presumably it is out of your pocket for longer. That raises a big question: how is it that testicular damage is supposed to occur? If you are holding it up to your head to speak a lot, it makes no sense that it is having a direct effect on your testes."

Other potential harmful effects

* CHILDREN: The chair of the Health Protection Agency, Sir William Stewart, said last year that children under eight should not be using mobile phones because there was still no certainty about the long-term health impact. The World Health Organisation said that "high priority" should be given to further research.

* BRAIN TUMOURS: Research this year found that regular users were not at greater risk of developing glioma - the most common type of brain tumour, although tumours were more likely to be diagnosed on the side of the head to which a handset was held.

* REPETITIVE STRAIN INJURY: Doctors have identified a growing number of people who are suffering from "text thumb" - a form of RSI caused by constant texting. In one case, a girl of eight needed treatment after she developed pain in her fingers and wrists.

* ADDICTION: Some people have developed a worrying psychological dependence on their phones, according to Australian research.

* BASE STATIONS: Rural residents may be more at risk because base stations emit more intense signals, Swedish scientists say.