Laboratory tests using leukaemia cells found genes that made the cells replicate far faster were turned on if they were exposed to the radiation for more than 48 hours.

We don't know what the effects would be on healthy human cells - but in leukaemia cells the response is always the same

Dr Fiorenzo Marinelli, National Research Centre, Bologna

It is still not clear whether this test tube experiment can apply to the real world - no reliable link between mobile phone users and a higher rate of cancers has yet been found.

The experiments, reported in New Scientist magazine, were carried out at the National Research Council in Bologna.

Leukaemia cells were exposed to radio waves at the 900-megahertz standard used by many GSM networks.

Suicide genes

The power used was 1 milliwatt, although it is hard to work out how much is absorbed by the tissues of humans using phones.

After 24 hours of continuous exposure, the leukaemia cells were responding by activating "suicide genes" - 20% more exposed cells were dying compared with control cells given no dose of radiation.

However, a day later, it was a different story.

The bottom line is there are no known mechanisms by which mobile phone radiation can increase the risk of cancer

Dr Zenon Sienkiewicz, NRPB

Instead, three genes that triggered the cancer cell to multiply rapidly had been switched on in a high proportion of surviving cells.

The cancer effectively became more aggressive as a result.

Professor Fiorenza Marinella told New Scientist: "We don't know what the effects would be on healthy human cells - but in leukaemia cells the response is always the same."

However, other studies into the effect of mobile phone radiation suggested that it had no influence on cell death.

Close watch

Dr Zenon Sienkiewicz, a radiation biologist at the National Radiological Protection Board (NRPB), told BBC News Online that there was still no hard evidence that showed mobile phones causing harm in real humans, rather than human cells in a test tube.

He said: "The bottom line is there are no known mechanisms by which mobile phone radiation can increase the risk of cancer."

He said that a risk could not be ruled out, which was why a recent inquiry report into mobile phone safety recommended a "precautionary approach" - particularly among children, with parents advised to restrict their mobile phone use to a minimum.

The NRPB is currently appraising all the latest research into the health effects of mobile phones and is due to report next year.

Dr Sienkiewicz said: "If we receive information on this research, we will of course examine it closely."