Motorists who use hands-free mobiles phones whilst driving have been likened to drink drivers because they suffer the same level of reduced concentration.
A study shows that doing so reduces concentration by more than a third.
And that loss of focus is said to lead to the same mistakes that drink-drivers make, such as weaving between lanes.
The study, by scientists at an American university, examined volunteers using a driving simulator while sitting inside a brain scanner.
Last month, the RAC Foundation found that nearly half of motorists regularly flout the law by texting while driving and a quarter have talked on a hand-held mobile.