People who use mobile phones regularly in rural areas are three times more likely than city dwellers to suffer from brain tumours, a study has found. Scientists believe that rural users of mobile phones receive relatively large doses of microwave radiation from their handsets to compensate for the fact that base stations in the countryside are further apart than in the city.

The findings are based on a sample of 1,400 patients with brain cancer who were compared against a further 1,400 healthy people who had also been interviewed about their use of mobile phones.

But the scientists who conducted the research admitted that the overall number of cases involved was still small and said the findings did not prove that using mobile phones can cause brain tumours.

Professor Lennart Hardell, a cancer specialist at the University Hospital of Orebro in Sweden, said the results of the study nevertheless pointed to a link between the dose of microwave radiation from a mobile and the risk of developing brain tumours.

"It's another piece of evidence, but of course we have to wait for further studies. This is a further step indicating that there is probably a problem and people should use the precautionary principle to limit their use of mobile phones, especially for children," Professor Lennart said.

The study, published in the journal Occupational and Environmental Medicine, investigated more than 1,400 Swedes aged between 20 and 80 who had been diagnosed with a malignant or benign brain tumour between January 1997 and June 2000.

The scientists found no link between the probability of developing a tumour and the time spent on the phone, but they did find a link between the risk of brain cancer and place of residence - rural or urban.

Residents of rural areas who had been using a mobile digital phone for more than three years were three times more likely to be diagnosed with a brain tumour than those living in urban areas.

For those rural residents who had used a mobile digital phone for five years or more, the risk quadrupled compared to city dwellers. Yet the scientists found no such increased risk when they looked at older, analogue mobile phones.

Professor Hardell suggested the reason was that digital phones use a system called adaptive power control, which automatically boosts the power output of the handset signals when base stations are located farther away.

Radiation emissions from a mobile phone handset can be 10 times higher in rural areas than in urban districts to compensate for the fact that base stations are located further apart, he said. "With analogue phones the emissions are constant and we did not see this difference between rural and urban areas," Professor Hardell said.

For malignant tumours the difference was even greater, with rural residents running an eightfold increase in risk compared to those living in urban areas.

But Professor Hardell said the absolute numbers involved were small and said that the findings must be treated with caution until further, large-scale studies were completed.

"The message is that people should use hands-free sets and limit their phone calls if possible," he said.

A spokesman for Britain's Health Protection Agency said Professor Hardell's study was not designed to test the hypothesis that rural phone use was more dangerous than in the city.

"We do need to be precautionary about the use of mobile phones, especially by young children," the spokesman said. "We also need to be precautionary about this study, because other research has not found a clear link between mobile phone handsets and brain cancers."

"We should wait for the results of the Interphone study which is being carried out at the moment. It is a large study in 13 countries and should give a good indication of whether or not there is a real cancer risk from mobile phone use."

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