The proportion of motorists using hand-held mobile phones while driving has jumped by 27 per cent, casting serious doubt on the effectiveness of enforcement against law-breakers.

Over the past year the number of drivers using phones increased from 1.1 per cent to 1.4 per cent for car drivers, and from 2.2 per cent to 2.6 per cent for van and lorry drivers, according to figures from the Department for Transport.

The Institute of Advanced Motorists said that motorists were clearly ignoring advice about using hand-held phones while driving.

The RAC Foundation, too, called for more enforcement. "This is very worrying," said Foundation director, Stephen Glaister. "We know that drivers' reaction times slow by almost half when they are having a chat on their mobiles.

"This is even worse than texting while driving – which is bad enough in itself – which reduces reactions by a third. It seems a small but growing minority of drivers choose to flout the law. Police must be given the resources to tackle this menace and divers persuaded that what they are doing is potentially lethal."