Prof Rony Seger, a cancer researcher at the Weizmann Institute of Science in Rehovot, Israel, and colleagues exposed rat and human cells to electromagnetic radiation at a similar frequency to that emitted by mobiles but at only about one tenth of the power.

After just five minutes the researchers identified the production of extracellular signal-regulated kinases (ERK1/2) - natural chemicals that stimulate cell division and growth.

Cancers develop when the body is unable to prevent excessive growth and division of cells in the wrong place.

Prof Seger said yesterday: "The real significance of our findings is that cells are not inert to non-thermal mobile phone radiation.

"We used radiation power levels that were around one tenth of those produced by a normal mobile. The changes we observed were clearly not caused by heating."

The UK has adopted international safety standards for electromagnetic radiation. These state that the amount of energy absorbed from an electric field or radio wave cannot exceed two watts per kilogram (W/kg) when averaged over 10 grams of tissue. Almost all mobile phones emit less that than one W/kg.

Graham Philips, of Powerwatch, a lobby group that campaigns on mobile phones, masts and powerlines, said: "Current safety guidelines assume health effects from mobiles can occur only when significant heating of body tissue occurs.

"This study shows biological changes in response to low-level mobile phone radiation - something that could have implications for health. Further research is required. However, guidance based purely on thermal effects is clearly out of date."

Other scientists pointed out that cell division occurred naturally as tissue grew or rejuvenated within the body, and that the preliminary study did not prove any health effects.

Simon Cook, a biochemist at the Babraham Institute near Cambridge, said: "The reason people are intrigued is that this pathway is frequently activated in cancer.

"The research is certainly interesting. However, they saw a very transient activation of this pathway, which we know is not sufficient to promote cell division.

"In cancer you see a much stronger, persistent and sustained activation and even this is just one of many changes required for cancer development."

Simon Arthur, from the University of Dundee, said: "The ERK1/2 pathway can be turned on by a huge variety of different things such as natural compounds produced by the body that regulate cell growth, and various forms of environmental and chemical stress.

"The research shows the effect on cells in culture in tightly-controlled laboratory conditions. In a living person there are lots of different processes occurring at the same time, so we do not know whether the signal from radio waves would produce a similar measureable effect."

The Health Debate

• May 2000: Parents left confused after an official report, chaired by Sir William Stewart, then chairman of the National Radiological Protection Board (NRPB), concluded that there were no proven health risks associated with mobile phones but that children should minimise their use as a precaution.

• Feb 2001: The Mobile Telecommunications and Health Research Programme (MTHR) set up to encourage further research into potential health hazards of handsets and masts.

• May 2001: Research in America suggested children's brains absorbed 50-70 per cent more radiation from handsets than adults because their skulls were smaller.

• Oct 2004: Swedish research concluded that those who used mobiles for 10 years were almost twice as likely to develop an acoustic neuroma - a tumour on a nerve connecting the ear to the brain.

• Jan 2005: Chairman of the Health Protection Agency advised parents not to allow children under nine to use mobiles because of potential but unproven risks.

• Dec 2006: A Danish study of people with brain tumours concluded there were no increased risks for heavy users.

• Jan 2007: A study in Finland of people with nervous system tumours called gliomas found no link with mobile use until it separated out long-term, regular users. It was concluded that they were 39 per cent more likely to get a glioma on the side of their head where they held their handset.

• Sept 2007: MTHR expected to present final report, including results of several unpublished studies. Prof Lawrie Challis, the chairman, expected to say there are no proven risks from short-term use, but to announce large-scale monitoring of health of handset users over 10 years.

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