A working group of experts has concluded that mobile phones could "possibly" cause brain cancer.
Members of the International Agency for Research on Cancer met in Lyon, France in May.
The 30 scientists from 14 countries delivered a cautious verdict on "limited" evidence from a number of human and animal studies.
Most supported the view that radio frequency electromagnetic fields (RF-EMF) were "possibly carcinogenic to humans". However, a few insisted the evidence was not strong enough.
The group's findings were summarised online today in The Lancet medical journal.
One large study, Interphone, published last year, found that overall, mobile phone use was associated with a slightly lower risk of developing a glioma, or brain tumour.
But focusing on the top 10pc of heavy users -- those with total exposure times of 1,640 or more hours -- revealed a 40pc increased risk of glioma. Glioma risk increased with elevated levels of exposure over periods of time of seven years or more.
The working group also looked at a combined analysis of Swedish studies which showed that using a mobile phone for more than one year led to a 30pc increased risk of glioma.
The risk level rose as more time elapsed since first use, and also with total call time. More than 2,000 hours of use was associated with a threefold increased risk.
"Although both the Interphone study and the Swedish pooled analysis are susceptible to bias -- due to recall error and selection for participation -- the Working Group concluded that the findings could not be dismissed as reflecting bias alone, and that a causal interpretation between mobile phone RF-EMF exposure and glioma is possible," wrote the scientists.
A further Danish study found no evidence of higher brain tumour rates among mobile phone users.
"A few members of the Working Group considered the current evidence in humans 'inadequate'," wrote the scientists, led by group chairman Dr Jon Samet, from the University of Southern California, US.
However the judgment that mobile phone radiation was "possibly carcinogenic" was "supported by a large majority of Working Group members".