Radiation emissions absorbed by the head from using mobile phones are cut by using hands-free kits, a study says.
The researchers were not looking at the safety of mobile phones
The University of York tested mobile phones which operated at two different frequencies and found using a hands-free kit cut emissions by 53%.
The team did not look at whether the phones were harmful but recommended using the kits as a precaution.
Government adviser Sir William Stewart said last week children should only use mobile phones in an emergency.
Sir William's report, five years after his first study, said there was still no evidence mobile phones were dangerous.
But Sir William, the chairman of the National Radiological Protection Board, said that should not stop parents being cautious.
The University of York researchers, funded by the government, used a model human head containing a liquid, which measured the specific absorption rate (SAR), and a wire-based hands-free kit.
The SAR is the rate at which the body absorbs emissions from the phone handset.
Most phones emit radio signals at SAR levels of between 0.5 and 1 watts per kg.
Mobile phones cannot be sold to unless they fall within the SAR of 2 w/kg.
Their findings come after conflicting reports on the effects of hands-free kits with some saying they reduced emissions, while others claimed they had little effect.
Dr Stuart Porter, of the department of electronics at the University of York, said one of the problems with previous studies was that they tended to rely on equipment that was not commercially available.
"From our perspective, we are not saying use hands-free kits because mobile phones are unsafe, but if people are concerned about potential risks there may be some value from a precautionary approach."
But a spokeswoman for the Mobile Operators' Association said there was no need to use hands-free kit on safety grounds.
"All mobile phones sold in this country comply with international health and safety exposure guidelines."
But she said if people did have concerns they could make a "personal" choice to use one.
Dr Michael Clark, of the National Radiological Protection Board, welcomed the findings, saying they "laid to rest the myth" that hands-free kits do not have an effect.