The largest-ever study into the possiblility of links between the use of mobile phones and brain cancer has proved inconclusive, the World Health Organisation (WHO) has said.
The WHO's Interphone study found there was "suggestions of higher risk" for those who spoke on their phones for more than half an hour a day on the same side of their head over a 10-year period.
The report said that these "heavy users" had a 40 per cent higher risk for glioma tumours and 15 per cent for meningioma tumours, but the researchers said "biases and errors" prevented them from making a causal link.
Given that using a mobile phone on average for 30 minutes per day is not heavy by today's standards, the researchers recommended further research.
"The study doesn't reveal an increased risk, but we can't conclude that there is no risk because there are enough findings that suggest a possible risk," Elisabeth Cardis, the study's chief author, said.
The report by the UN agency looked at both healthy users of mobile phones and those with brain cancer, including 2,708 cases of glioma tumours and 2,409 cases if meningioma tumours.
The study took place in 13 countries over a 10-year period, but was controversial because mobile phone companies put up 25 per cent of the funding.
The researchers cited the need for the study of the impact of mobile phone use among young people, who have rapidly become intensive users, and who were not included in the Interphone study.
"Observations at the highest level of cumulative call time and the changing patterns of mobile phone use ... particularly in young people, mean that further investigation of mobile phone use and brain cancer is merited," Christopher Wild, the director of the International Agency for Research on Cancer, which co-ordinated the study.
The study noted, however, that the latest mobile phones have lower emissions, and the popularity of hands-free devices and texting reduce exposure to the head.
Cardis said the European Union is funding a new study of risks of brain tumours from mobile phone use during childhood and adolescence.
Last month, Imperial College in London began the world's largest study into the health effects of mobile phones which will see 250,000 mobile phone users have their medical records tracked for more than 20 years.