Researchers found using a mobile in rural areas tripled the risk of malignant or benign tumours compared to urban users.

But the industry said the findings were not backed up by recent research.

The study by University Hospital in Orebro was published in Occupational and Environmental Medicine journal.

Several studies have looked into the safety of mobile phones in recent years with mixed results.

Experts in the UK have indicated there is no proven risk, but the Health Protection Agency has called for children to only use them when necessary.

The Swedish study looked at 1,400 adults aged 20 to 80 who had been diagnosed with a malignant or benign brain tumour and compared them to healthy adults living in the same area.

Using analogue or cordless phones did not have an effect, the researchers found.

But they found country residents using digital mobiles were three times as likely to develop a tumour compared to urban users, who had about the same risk of brain tumours as the general population.

For malignant brain tumours, the risk was eight times as high for those living in a rural area, but the numbers were very small, the researchers warned.

Lead researcher Professor Lennart Hardell said the cause of the increased risk seemed to be the higher emissions from the phones in rural areas because the base stations were further apart than they would be in cities and towns.

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But emissions can also be higher in urban areas where reception is poor, and this too can increase the tumour risk.

Analogue phones have the same emissions wherever they are used.

More research

Professor Hardell said: "My advice would be to operate the precautionary principle. Use hands-free kits when possible and children should only be using mobiles when necessary," Prof Hardell said.

"What we need is for future research to look into what effect residential settings have."

Dr Michael Clark, a radiation expert at the Health Protection Agency, said it was an interesting paper, but involved only a small number of people.

"Because of uncertainties in our knowledge about this widely-used technology, we have recommended a precautionary approach on the use of handsets.

"People, particularly children, should minimise their usage until research is properly evaluated over the next few years."

Mike Dolan, executive director of the Mobile Operators Association, said: "At best this study is hypothesis generating.

"Its findings are not in line with the most recent epidemiological studies from Denmark and Sweden which have not found an association between brain tumours and mobile phone use."

And he added all handsets in the UK complied with health and safety guidelines.