A decade-long investigation by the World Health Organisation (WHO) is expected to publish evidence showing that heavy users are at greater risk of developing brain tumours later in life.
Such a conclusion contradicts assurances from the UK government that mobiles are safe, and is likely to prompt calls for stronger guidance.
Previous research carried out into the health effects of mobile phones has proved inconclusive.
But details of the 20 million investigation published yesterday show that a preliminary breakdown of the results found a "significantly increased risk" of some brain tumours "related to use of mobile phones for a period of ten years or more" in some studies.
The Interphone investigation has been probing whether exposure to mobile handsets is related to three types of brain tumour and a tumour of the salivary gland.
Dr Elisabeth Cardis, head of the study, said the report would include a "public health message" and she recommended the use of mobiles by children be curbed.
She said: "In the absence of definitive results and in the light of a number of studies which, though limited, suggest a possible effect of radio frequency radiation, precautions are important.
"I am therefore globally in agreement with the idea of restricting the use by children, though I would not go as far as banning mobile phones as they can be a very important tool, not only in emergencies, but also maintaining contact between children and their parents and thus playing a reassurance role."
The investigation comprises studies across 13 countries between 2000 and 2004, with questions put to 12,800 people. Six out of eight of the studies have found some rise in the risk of glioma – the most common brain tumour – with one finding a 39 per cent increase.
Two of seven studies into acoustic neurinoma, a benign tumour of a nerve between the ear and the brain, reported a higher risk after using mobiles for a decade.
The overall investigation has been delayed amid arguments over how to present its conclusions, but it has been sent to a scientific journal and is expected to be published before the end of the year.
The study, funded partly by the industry, has been criticised for including people who made just one mobile call a week, and leaving out children, which some said could underplay the risks.
Some results for short-term use, meanwhile, appeared to show protection against cancer, suggesting flaws in the study.
The Department of Health has not updated its guidance on mobiles for more than four years. It claims that "the current balance of evidence does not show health problems caused by using mobile phones". A spokesman for the Health Protection Agency said that there was "no hard evidence at present" of mobile phones being harmful, but said their use by children for non-essential calls should be discouraged.
A spokesman for the Mobile Operators Association said that more than 30 scientific reviews had found no adverse health effects.