Mobile phone experts say their research gives no clear answer whether mobile phone devices cause brain tumours.
A study by the World Health Organisation's International Agency for Research on Cancer (IARC) looked at 13,000 cell phone users over 10 years.
It is the largest ever to investigation into possible links between mobile phones and brain cancer. Despite it throwing up inconclusive results researchers say suggestions of a possible link demand 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," said the IARC's director Christopher Wild.
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.
Data from the IARC study also 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.