Ronald Herberman, of the University of Pittsburgh Cancer Institute, said users should not wait for definitive studies on the risk and should take action now.  After sending a memo warning to 3,000 staff says children should be protected as their brains are still developing. They should only use mobiles in emergencies only and adults should try to keep the phone away from the head.

Dr Herberman said his warning was based on early findings from unpublished data.

"We shouldn't wait for a definitive study to come out, but err on the side of being safe rather than sorry later," he says. "I am convinced that there are sufficient data to warrant issuing an advisory to share some precautionary advice on cell phone use," the memo says.

He lists tips including switching sides regularly while talking on mobiles.

A major six-year research study in the UK said last year that there were no short-term adverse effects to brain and cell function from mobile phone use. However, the UK Mobile Telecommunications and Health Research Programme said there was a "hint" of a higher cancer risk in the long term and that its research would look into the effects over a 10-year period.

Mobile phones emit radio signals and electromagnetic fields that can penetrate the human brain, and some campaigners fear that this could seriously damage human health.

A US analysis by the University of Utah this year of thousands of brain tumour patients found no increased risk as a result of mobile use, but added that the effects from long-term use "awaits confirmation by future studies".

Also research in 2006 by the British arm of an international project called Interphone concluded that mobile phone use did not lead to a greater risk of brain tumour, but a study of 500 Israelis found this year that heavy mobile phone use might be linked to an increased risk of cancer of the salivary gland.