People who have used mobile phones for 10 years or more have an increased risk of brain tumours, three European research groups have found.
The separate studies add new fuel to the debate over mobile phone safety.
A correlation between the tumour's location and the side of the head where people reported they held the phone was found in two of the studies.
One also suggests the greatest risk is in people who began using the phones before age 20. Researchers said the study group was small and more research should be done.
Two of the studies, one in England and one in Germany, are part of the 13 nation Interphone Study, an effort sanctioned by the World Health Organisation to assess possible health risks from the radiation emitted by mobile phones.
Both studies found an increased risk of glioma, an often deadly brain cancer, in people who had used mobile phones 10 years or more.
An earlier Interphone study, reported in October 2004 by researchers at the Karolinska Institute in Sweden, found an increased risk for a non-cancerous brain tumour called acoustic neuroma after 10 years of mobile phone use, but not for glioma.
"When you put the three large Interphone results together - the German, English and Swedish - they tell a story, and it begs for attention," said Louis Slesin, publisher of Microwave News, who has been reporting on the health effects of such radiation for two decades.
John Walls, vice president of public affairs for CTIA, The Wireless Association, a mobile phone industry trade group in Washington, said the increase in glioma in people who had used the phones more than 10 years was "statistically insignificant".
There is no cause for concern, he said.
The German study, conducted by Joachim Schuz and colleagues at the University of Mainz, was published online by the American Journal of Epidemiology.
The researchers compared a group of 749 brain tumour patients with 1,494 similar people who had not used mobile phones and found a doubling of the risk of glioma after 10 years of use.
They said the number of people in the study who had used the phones for 10 years was small, and the findings needed to be confirmed by other studies.
The British researchers compared a group of 966 brain tumour patients with a group of 1,716 healthy patients who had not used mobile phones.
They found a 20 per cent increase in cancers among long-term users, but no overall increased risk in people who used mobile phones.
The study, funded largely by the mobile phone industry and published online by the British Medical Journal, found a significantly increased risk for tumours that developed on the side of the head to which patients said they most often held the phone.
But lead researcher Patricia McKinney said that finding probably was due to many patients not accurately recalling which ear they'd used most of the time.
Critics said conclusions drawn by the researchers were "highly misleading" and might give mobile phone users a false sense of security.