More doubt has been cast on the safety of mobile phones by a new study which says men who use them could be risking their fertility.
The researchers warn that they have found a link between poor sperm and the number of hours a day that a man uses his mobile phone.
It seems that those with the worst sperm counts and the poorest quality sperm, spend more than four hours a day on a mobile phone.
Professor Ashok Agarwal and colleagues at the Cleveland Clinic Lerner College of Medicine at Case Western Reserve University in Ohio, say the damage could be caused by the electromagnetic radiation emitted by handsets or the heat they generate.
The findings are a concern because they suggest millions of men may be putting their chances of becoming a father at risk by the widespread use of mobile phones.
It is also of concern because sperm counts among British men have fallen by 29 per cent over the past decade, which is usually blamed on increasing obesity, smoking, stress, pollution and 'gender-bending' chemicals which disrupt the hormone system.
This latest study supports previous research which also indicated a link between mobile phone use and sperm quality, but is to date the biggest and best designed study.
In a collaborative effort, researchers in Cleveland and New Orleans in the U.S., and doctors in Mumbai, India, examined data on more than 360 men undergoing checks at a fertility clinic who were classified into three groups according to their sperm count.
The team found that men who used a mobile for more than four hours a day had a 25 per cent lower sperm count than men who never used a mobile.
These men also had poorer quality sperm, with swimming ability - a crucial factor in conception - down by a third.
They also had a 50 per cent drop in the number of properly formed sperm, with just one-fifth of normal appearance under a microscope.
Professor Ashok Agarwal, the study leader, says almost a billion people use cell phones around the world and the number is growing in many countries at a rate of 20 to 30 per cent a year.