Biologist Roger Coghill has long campaigned for health warnings to be attached to mobile phones, which he has already linked to headaches and memory loss.

His latest research suggests the microwaves generated by mobile phones may damage the ability of white blood cells to act as the "policemen" of the body, fighting off infection and disease.

Mr Coghill took white blood cells, known as lymphocytes, from a donor, keeping them alive with nutritients and exposed them to different electric fields.

He found that after seven-and-a-half hours, just 13% of the cells exposed to mobile phone radiation remained intact and able to function, compared with 70% of cells exposed only to the natural electromagnetic field produced by the human body.

Body's balance is upset

The dark cell is a lymphoctye - the body's policeman

Mr Coghill claims the body's immune system is partially controlled by electromagnetic fields emitted by the body. He believes the radiation emitted by mobile phones damages the body's own electromagnetic fields, and undermines the proper functioning of the immune system.

Mr Coghill has launched a legal test case against a mobile phone shop for allegedly failing to warn customers of the potential risk of radiation.

The industry is worth a £14bn a year in Britain alone.

Industry attacks findings

Tom Wills-Sandford defends mobile phones

Mr Coghill was criticised by a leading industry figure for not announcing his findings before they had been reviewed by experts and published in a recognised scientific journal.

Tom Wills-Sandford, director of the Federation of the Electronics Industry, which represents mobile phone manufacturers, said: "None of the proper scientific protocol has been followed.

"This is not a proper way to conduct science, and one wonders if these results will ever be published properly."

Mr Wills-Sandford said an enormous amount of research had been carried out into the safety of mobile phones but none had produced any real evidence of a risk to health.

'Scientifically sound'

But Mr Coghill, who spoke at a conference on mobile phone safety in London on Thursday, insisted that his results were scientifically sound and should not be ignored.

He said: "We found that the competence of these white blood cells was depleted after being exposed for seven or eight hours to a mobile phone on standby.

"There's a possibility that we are damaging lymphocyte performance simply by having these phones on standby next to our bodies."

Mr Coghill said there was no danger in using mobile phones for two or three minutes.

But people who left them on for 20 minutes or more could be doing themselves harm.

If even 5% of the estimated 10 million users left their phones switched on it would mean 500,000 people were at risk, he said.

Mr Coghill said: "What I'm asking for is that the industry recognises that and puts warning labels on their phones."

He said a paper on his findings was accepted for inclusion at a major scientific meeting in Florida, USA, in June.

He was also going to be forwarding the results to a recognised journal and co-operating with other scientists trying to replicate the findings.