Using a mobile phone for 10 years or more increases the risk of ear tumours by four times, research suggests.
Using a mobile phone for at least 10 years increases the risk of ear tumours, scientists believe
A 750-people study by Sweden's Karolinska Institute found the risk of acoustic neuroma rose by 3.9 times on the side of the head the phone is used.
There was no increase in risk on the other side of the head - giving an overall rise in risk of 1.9 times.
Acoustic neuroma is a benign tumour in the auditory nerve, which can cause brain and nerve damage.
It affects one in 100,000 people.
Those who had used mobile phones for less than 10 years were not at a greater risk, the team reported.
Out of the 750 people who took part in the study, 150 had acoustic neuroma and of those one in 11 had used a mobile phone for at least a decade.
Professor Anders Ahlbom, from the Stockholm-based institute, told BBC News Online he was "surprised" by his team's findings.
"The results show there is a relatively substantial risk and we are hoping others will follow up our research.
"We do not know what is causing it but the risk certainly increases over time."
He said he would not go as far as warning people not to use mobile phones.
But he added: "If people are concerned the simple way to avoid risk is to use a hands-free kit.
"Our researched showed that the risk is only on the side of the head on which the mobile phone is used."
At the time of the study only analogue mobile phones had been in use for more than 10 years.
The majority of people now have digital (GSM) phones, which came on to the market in the mid to late 1990s.
Some of the people who took part in the study had used both analogue and digital phones. There was no evidence to suggest solely using digital phones for 10 years increased the risk.
Dr Michael Clarke, a spokesman for the National Radiological Protection Board, the UK's advisory group, said it was a "good study from a well respected institute".
He said: "It is suggestive rather than conclusive but we will obviously take it into account when we issue guidance in the future."
And a spokeswoman for the Mobile Operators Association said: "The mobile phone industry takes very seriously questions relating to the safety of its products and is committed to addressing public concern in an open and transparent manner."
But she added: "Individual studies must be seen in the light of the total research effort into mobile phone safety. There have been other recent studies that have failed to show any link between mobile phones and tumours."
Three quarters of adults in the UK own or use a mobile phone.
The mobile phone industry has always maintained there is no scientific evidence of negative effects from mobile phone use.
But over the last few years experts have remained divided over the question of risk.
A study by Finnish scientists in 2002 found electromagnetic radiation, which is emitted from mobile phones, affected human brain tissue.
But the UK government-commissioned Stewart report in 2000 concluded there was no evidence of harm associated with using mobile phones.
However, the report recommended a precautionary approach and said children should only use mobile phones in emergencies.