Manuals for mobile phones tell users not to place devices directly against their bodies but the advice is often buried in small print.
Research into the potential danger caused by mobiles has proved inconclusive, but some studies have suggested an effect on the brain and on sperm quality.
Alasdair Philips of Powerwatch, an independent group that investigates the safety of mobile phones, said: "Most people have no idea about these warnings.
"The advice should be included on the boxes and far more prominently in the 'getting started' section of user guides and not just in the detail at the back that hardly anyone reads," Philips told the Daily Mail.
Meanwhile, Devra Davis, a leading US scientist, has warned that if the health risks were ignored, there could be a "global public health catastrophe" within three years.
Mobile phones are low-powered radio frequency transmitters that produce microwave radiation.
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