An official government inquiry into the health risks of mobile phones will confirm next week that the single biggest hazard is talking on them while driving.

An official government inquiry into the health risks of mobile phones will confirm next week that the single biggest hazard is talking on them while driving.

The eight-month investigation by a committee of 12 scientists and laymen has found no evidence to link the radiation of cellphones with brain tumours, loss of memory or any other health problems.

However the committee, chaired by Sir William Stewart, a former government chief scientist, has received evidence showing that having a conversation on a cellphone while driving a vehicle can result in a fourfold increase in the chances of having an accident.

The increased risk occurs irrespective of whether a driver is using a "hands-free" cellphone. Scientists told the inquiry it is the act of speaking to somebody who is not actually in the car that creates intense distraction for a driver.

Three separate investigations into driving and mobile phone use - two in Britain by the Royal Society for the Prevention of Accidents (Rospa), and one in Canada - have all produced statistically significant evidence to link cellphones with traffic accidents. The Canadian study on 699 drivers involved in traffic accidents found that those who used cellphones were 4.3 times more likely to have a crash - roughly equivalent to the risk of driving while just above the legal limit for alcohol.

Dave Rogers, Rospa's road safety adviser, said: "It is the distraction of the conversation rather than the mechanics of using the phone that posesthe real problem. Talking to a passenger in the vehicle does not cause the same level of distraction."

The Stewart committee's recommendations will be published next Thursday and are expected to say that while there is no evidence of health risks - other than talking while driving - further research is still necessary to rule out completely all possible ill-effects of microwave radiation from cellphones.

Current controls on microwave emissions are based on the assumption that the radiation can only affect living tissue by heating. However, the Stewart committee is expected to warn about possible "non-thermal" effects of mobile phones which are at present little understood.

Test-tube experiments on cells and tissues have suggested that low levels of microwave radiation - too low to cause heating - can produce notable biological disturbances, but scientists have so far been unable to demonstrate similar effects in laboratory animals.

The Stewart committee is expected to report that although it is concerned about possible non-thermal effects of low-level microwaves, there is no evidence as yet to show that the radiation levels emitted by cellphones can cause any adverse health effects.

The committee is likely, however, to recommend further avenues of research, such as long-term epidemiology studies of cellphone users. "Although there was no evidence of a relationship between cellphone radiation and illnesses such as cancer, there was not enough evidence to dismiss the link altogether," said one scientist close to the committee.