The BBC's Pallab Ghosh: Using mobiles seemed to increase the speed of response in the test

Public health minister Tessa Jowell said the government had set up a panel of experts to review all research on mobile phones.

The panel will be part of the National Radiological Protection Board, which provides independent advice on the risks to health from radiation.

She said: "To date there has been no consistent evidence suggesting risk to health, but there is continuing public concern about the possibility."

"I believe we need a definitive and rigorous assessment of existing research and clear identification of areas where further research may be needed so that the public can receive clear advice about the use of mobile phones and a clear risk assessment from independent experts."

The government move came as the first evidence emerged that mobile phones affect the functioning of the human brain.

Quicker reactions

Government-funded research by the Bristol University and Bristol Royal Infirmary found that mobile phones speeded up users' reactions by about 4% when they were faced with questions requiring a yes or no answer.

This could be because they heat the brain - which would be unlikely to have any long-term effects - or because they cause changes associated with a threat of disease.

Several previous studies have suggested that mobile phones may be harmful to human health, but there is not yet any consistent evidence that this is the case.

Some have suggested that the devices may cause short-term memory loss.

Exposure to signals

The Bristol study of 36 healthy volunteers found that there was no proof of any effect on short-term memory or attention from exposures of up to 30 minutes.

The BBC's Pallab Ghosh: 11m in the UK now have mobiles

The 36 were exposed to microwaves typical of analogue phones, microwaves typical of mobile phones and no microwaves at all.

However, at no time did they know to which type of microwave they were being exposed.

When exposed to mobile phone microwave levels, they were put in a "near worst case" scenario.

Microwave exposure from mobiles varies according to the strength of the signal received.

In areas of good reception, the microwave dose is light.

The users in the tests were given a strong dose.

The volunteers were put through a series of tests, including reaction times, spatial memory, immediate word recall and delayed word recall.

The only major difference recorded was in reaction times to yes and no questions.

The researchers think there are two possible reasons for the variation - that the mobile users' brains are temporarily heated leading to increased blood flow or that a change occurs in protein synthesis, leading to the creation of heat shock proteins.

These proteins are usually caused by the body's defence mechanisms and are a reaction to a threat of disease or damage from an injury.

Heat shock proteins

Writing in the International Journal of Radiation Biology, the researchers, led by Dr Alan Preece, say their study shows that mobiles affect the brain.

Dr Alan Preece explains how the research worked

"Such changes may be neither harmful nor permanent, provided any mechanism is a temperature effect below the level for heat shock protein production and no other non-thermal response," they say.

The researchers want more studies into the long-term effects of mobile phones on heavy users.

They have begun further research into what causes the faster reaction times.

They also warn that any impact on the brain of using mobile phones may be greater on children than adults.

Dr Preece said: "The amount of space between the skull and brain is smaller in children so they possibly may be more at risk from using phones."

The Federation of the Electronic Industry (FEI) welcomed the publication.