The report, in the Daily Telegraph, says the conclusion was not definitive but would undermine British government assurances that mobiles are safe.
The Interphone investigation cost £20 million ($A35.36 million). A preliminary breakdown of its results said a "significantly increased risk" of some brain tumours "related to use of mobile phones for a period of 10 years or more” was found in some studies.
The study has not been formally released and its head, Dr Elisabeth Cardis, told the newspaper that a public health message would be included.
Studies were done in 13 countries, and both people with tumours and those in good health health were interviewed to find out their mobile use patterns. A total of about 12,800 people were interviewed between 2000 and 2004.
The paper reported that six of eight Interphone studies had discovered a rise in the risk of suffering a glioma (the most common brain tumour). One found an increase of 39 per cent increase.
The final paper is likely to be published before the end of 2009 and has been held up as the authors are debating over how they should present their findings.
Last year, the French Health Ministry issued a warning against excessive mobile, use, especially by children. The issue is an emotive one and often the answer depends on whom one asks .