People, who use a mobile phone in rural areas, may face a greater risk of developing a brain tumour than those living in towns and cities, new research suggested.
The study in Sweden found that using a mobile phone in the countryside appeared to make people more likely to be diagnosed with a tumour than those living in urban areas.
Experts in the UK have indicated that there is no proven health risk to using mobile phones. But they have acknowledged the possibility of a problem emerging after prolonged use as mobile technology was still a relatively new phenomenon.
Sir William Stewart, chairman of the Health Protection Agency, has called on parents to ban under-eights from using mobile phones and wants teenagers to restrict their use and rely more on sending text messages.
The latest study, published in Occupational and Environmental Medicine, looked at more than 1,400 adults, aged 20 to 80, in Sweden who had been diagnosed with a malignant or benign brain tumour.
The researchers, from University Hospital in Orebro, compared the group with a similar number of healthy adults living in the same area.
Each group was asked about their daily use of mobile and cordless phones and also about their employment history.
The researchers concluded that how long people spent on the phone had little impact on the probability of their being diagnosed with a brain tumour. But they did find a difference when taking into account where people lived, especially for digital mobile phones.
People living in a rural area, who had been using a mobile digital phone for more than three years, were more than three times as likely to be diagnosed with a brain tumour as those living in urban areas.
Digital mobile phone use for five years or more in a rural area quadrupled the risk compared to living in urban areas.
The researchers, led by Prof Lennart Hardell, noted that for malignant brain tumours the risk was eight times as high for those living in rural areas.
But they pointed out that the actual number they based the results on was very small and they should be interpreted with caution.
Past studies have indicated that there is a difference in the power output levels from mobile phones between urban and rural areas.
This is because base stations tend to be much further apart in the countryside and need a higher signal intensity to compensate.
The researchers said: "Clearly our results support the notion that exposure may differ between geographical areas. However, these results refer to Sweden and there is no information on the exact difference between different geographical areas."
Dr Michael Clark, science spokesman at the Health Protection Agency, said: "As the authors themselves say, the results should be interpreted with caution."