Using a mobile phone does not appear to increase the risk of developing certain types of brain cancer, the largest study of its kind has concluded.
Analysis of more than 10,000 people by the International Agency for Research on Cancer (IARC) found no relationship between years of use and risk.
There is no known biological mechanism by which mobiles could cause cancer, but there has been public concern.
It is hoped this study will allay some anxieties, as research continues.
The overall rate of brain cancer has not risen in countries where use has long been prevalent - like Sweden, and studies have mostly found no evidence of an increased risk. This latest research is consistent with this.
The 20m euro (£17m) Interphone study, which received some funding from the mobile industry, involved more than 5,000 men and women from 13 countries who had been diagnosed with one of two types of brain cancer - glioma and meningioma - between 2000 and 2004.
These cancers, both rare, were judged to be among the most likely to be influenced by phone use.
The patients were asked to record their mobile phone usage, and then the results were compared with adults of similar age, sex and background who did not have the disease.
Some had been using their phones for more than a decade, making this one of the longest-ranging studies to date.
Extreme results
In fact, most regular users - defined as people who made use of their phone at least once a week - appeared to have a lower risk of brain cancer than those who rarely used a phone. The report authors stressed however this was unlikely to be down to any protective effect of phone use, and more a quirk of the study.
But they also dismissed as problematic the finding that at the other extreme end of the spectrum those using the phone for the longest cumulative periods, more than 1,640 hours, appeared to have a higher risk, regardless of over what period of time this was spread. This was as much as 40% higher for glioma, and 15% higher for meningioma.
Nearly 50 cancer patients reported using their phone more than five hours every day of the week, with 10 recalling that they had used it for 12 hours each day.
"It's not impossible that people were using their phones for this long, but it is highly unlikely," said Professor Anthony Swerdlow of the Institute of Cancer Research, which carried out one of the two UK arms of the study.
The report noted that people with brain tumours were more likely to overestimate the role of a potential risk factor, and that the disease interferes with memory and cognition, undermining the accuracy of the recollections of such extreme use.
Unlike lung cancer, where the risk rises the more cigarettes are smoked, this mobile phone data shows no increased risk until the very heaviest use begins. This was also seen as casting doubt on the reliability of the reports.
"This study cannot answer whether there are long-term risks beyond fifteen years, nor would it have been able to pick up much, much smaller risks," says Professor Swerdlow. "But if there was a large and immediate risk we would have seen it.
"Whether it is worth doing more research, that is a question for society. These are expensive studies, and there are many other things in the world that should be investigated.
"It is society which has to answer the question of how long you continue to investigate something that does not have a biological basis."
There were however reports of some differences within the research team as to how best to interpret the data, and the authors do say that further analysis of long-term, heavy use is in order.
A prospective study was in fact launched last month to examine the long term effects of mobile phone use. Cosmos, the cohort study on mobile communications, will recruit 250,000 and follow them for as long as 30 years.
The lobby group Mast was not assuaged by the Interphone findings, arguing that it wanted to see results for other cancers including salivary gland tumours and acoustic neuromas.
