People who started using mobile as teenagers and have been doing so for more than a decade are at a five-fold risk of developing a common type of brain cancer, new evidence indicates.
The Swedish study found large increased incidence of astrocytoma, the most common form of a malignant brain tumour type called glioma, in those who had been using mobiles for over 10 years.
Campaigners said the research, published in the International Journal of Oncology, was further evidence of the need to educate children of the potential dangers of talking on mobile phones.
Researchers from the University Hospital of Örebro and Umeå University examined the mobile and cordless phone use of more than 1,200 Swedes, who were diagnosed with malignant brain cancer between 1997 and 2003.
Of those, the 905 who were still alive were interviewed about their phone usage. For the remaining 346 who had died., researchers asked their relatives about their loved-ones' telephone habits.
They then compared this to phone use information on almost 2,500 'controls' who were either living and had no brain cancer, or had died of other causes. Each 'case' and each 'control' was matched for age, sex and social class.
The team concluded that using both mobiles and cordless phones led to "an increased risk for malignant brain tumours".
People who started using mobiles as teenagers, and have done so for at least 10 years, were 4.9 times more likely to develop astrocytoma, compared to controls.
Worringly, the comparable figure for cordless home phones - which are very similar to mobiles in terms of radiation emission - was almost as high, at 3.9.
Looking at the whole group, regardless of age of first ise of mobile or cordless phone, they found that usage for more than 10 years increased the risk of all malignant tumours by 30 per cent, and astrocytomas in particular by 40 per cent.
The study comes weeks after the International Agency for Research on Cancer, part of the World Health Organisation, stated that radiation from handsets was "possibly carcinogenic", although it stopped short of declaring there was a clear link.
The Department of Health advises that "children and young people under 16 should be encouraged to use mobile phones for essential purposes only, and to keep calls short".
Alasdair Philips, director of the campain group Powerwatch, said studies like the Swedish one "highlighted the need to teach our children that mobile phone use can be very dangerous to their long-term health".
Instead of calling they should text or use "air-tube" hands-free headsets which eliminated the risk of radiation, he said.
Although an increasing number of people wear Bluetooth wireless headsets or standard wired earplugs, both still carry the risk of radiation - the former because it is itself a transmitter, and the latter because radiation can pass down the wire into the ear.
Air-tubes, such as the Air2Hear, cut radiation exposure to the brain to almost zero by replacing the last six inches of wire with a hollow tube down.
According to official cancer registeries, there are about 8,600 diagnoses of 'primary' brain tumour per year in the UK.
However, according to the Samantha Dickson Brain Tumour Trust, it is "widely accepted that these figures are an under-estimate of the actual numbers".
The true number could be almost double that, the charity fears.
A spokesman said: "The human cost is alarming. Malignant primary brain tumours take more years off the average person’s life than any other cancer. They are the most significant cause of cancer death amongst men under 45 and women under 35, and approximately 500 children are diagnosed with a primary brain tumour each year."
Neil Dickson, founder of the charity, said of the Swedish study: "The findings do give us cause for concern.
"We have contributed to a recent paediatric study in Europe and we are waiting the full results from this so together with reports such as this, it is clear to us that further research is urgently needed in this area."