Using a mobile phone for more than half an hour a day over five years can triple the risk of developing certain types of brain cancer, a French study suggests.
Researchers found that people who used mobiles for 15 hours per month on average had a two to three times greater risk of developing glioma and meningioma - the main types of brain tumour - compared with those who used their phone rarely.
The findings, by researchers at Bordeaux University, supported other international studies, suggesting a "higher threat of a brain tumour observed solely among heavier [mobile phone] users".
While a string of studies have failed to find conclusive proof of a link between mobile use and cancer, several have suggested intensive, long-term use can raise the likelihood of contracting gliomas.
Isabelle Baldi, one of the scientists who led the French research, published in the British journal Occupational and Environmental Medicine, said: "Our study is part of that trend, but the results have to be confirmed."
Researchers examined 253 cases of glioma and 194 cases of meningioma reported in four French departments, or counties, between 2004 and 2006.
These patients were compared with 892 controls, or healthy individuals.
The comparison found a higher risk among those who used their phone intensively, especially those who used it for their work, such as in the sales industry. The duration of use in this category ranged from between two and 10 years, averaging at five years.
There were, however, unexplained anomalies. In contrast with previous research, the study found that cancer occurred on the opposite side of the brain to where the phone was habitually used, rather than on the same side.
"It is difficult to define a level of risk, if any, especially as mobile phone technology is constantly evolving," the study conceded.
"The rapid evolution of technology has led to a considerable increase in the use of mobile phones and a parallel decrease of [radiowave intensity] emitted by the phones.
"Studies taking account of these recent developments and allowing the observation of potential long-term effects will be needed."
In 2011, the International Agency for Research on Cancer warned that radiofrequency fields used by mobile phones were possibly carcinogenic.
The largest study to date on the risks of mobile phone use, the 2010 Interphone study, found no raised risk of brain tumours among those who regularly used mobiles.