Experts who studied almost 13,000 cell phone users over 10 years, hoping to find out whether the mobile devices cause brain tumours, said on Sunday their research gave no clear answer.

A study by the World Health Organisation's International Agency for Research on Cancer (IARC), the largest ever to look at possible links between mobile phones and brain cancer, threw up inconclusive results but researchers said suggestions of a possible link demanded deeper examination.

'The results really don't allow us to conclude that there is any risk associated with mobile phone use, but... it is also premature to say that there is no risk associated with it,' the IARC's director Christopher Wild told Reuters.

The results of the study have been keenly awaited by mobile phone companies and by campaign groups who have raised concerns about whether mobile phones cause brain tumours.

Years of research have failed to establish a connection.   

Wild said part of the problem with this study, which was launched in 2000, was that rates of mobile phone usage in the period it covered were relatively low compared with today.   

It was also based on people searching their memories to estimate how much time they spent on their cell phones, a method that can throw up inaccuracies.   

European scientists last month launched what will now become the biggest ever study into the effects of mobile phone use on long-term health. It aims to track at least a quarter of a million of people in five European countries for up to 30 years.

This kind of study, called a prospective study, is considered more accurate because it does not require people to remember their cell phone use later but tracks it in real time.

Data from the IARC study showed that overall, mobile telephone users in fact had a lower risk of brain cancer than people who had never used one, but the 21 scientists who conducted the study said this finding suggested problems with the method, or inaccurate information from those who took part.

Other results showed high cumulative call time may slightly raise the risk, but again the finding was not reliable.

'We can't just conclude that there is no effect,' said Elisabeth Cardis of the Centre for Research in Environmental Epidemiology in Barcelona, Spain, who led the study.   

'There are indications of a possible increase. We're not sure that it is correct. It could be due to bias, but the indications are sufficiently strong... to be concerned.'    

Because of this, and because cell phone use is rising all the time, more research was needed, the scientists said.   

The 21 scientists were part of a group known as the Interphone International Study Group which was funded in part by money from mobile phone companies. The study was published in the International Journal of Epidemiology.

Mobile phone use has increased dramatically since its introduction in the early-to-mid 1980s. About 5 billion mobile phones are currently in use worldwide.

The researchers said the majority of people covered in their study 'were not heavy mobile phone users by today's standards'.
 The average lifetime cumulative call time for those who took part was around 100 hours, with an average of 2 to 2-1/2 hours of reported use a month. The heaviest 10 percent of users had clocked up an average of 1,640 hours of phone use spread over 10 years, which corresponds to about half an hour a day.

'Today, mobile phone use has become much more prevalent and it is not unusual for young people to use mobile phones for an hour or more a day,' the researchers wrote.

But increasing use is tempered by generally lower radiation emissions from modern phones and greater use of texting and hands-free sets that keep the phone away from the head, they said.

The study received 19.2 million euros ($24.4 million) in funding, around 5.5 million euros of which came from industry sources. It analysed data from interviews with 2,708 people with a type of brain cancer called glioma and 2,409 with another type called meningioma, plus around 7,500 people with no cancer.

Participants were from Australia, Canada, Denmark, Finland, France, Germany, Israel, Italy, Japan, New Zealand, Norway, Sweden and Britain.  -Reuters