Around a third (32%) of motorists admitted to making or receiving a call on a hand-held phone while driving last year, even though 82% knew it was illegal to do so.

As well as putting themselves and other motorists at risk, the habit is an expensive one costing drivers caught using their mobile illegally at the wheel £10 million a year in fines and higher insurance premiums.

Latest figures from the Police and Department of Justice reveal that more than 120,000 drivers have been handed fixed penalty notices or prosecuted in court over the past 12 months.

This brings the total number of offenders flouting the law during the past eleven years to one million.

However, research carried out for Halfords has found that the number of people breaking the law could be as high as nine million.

Worryingly among this group of motorists a hardcore (6%) of persistent offenders admitted to doing it at least once a week despite the fact that, as well as risking a fine, insurance companies can add a minimum 5% onto the annual premium and some refuse to quote at all.

Dave Poulter, Halfords auto category director said: “Breaking the law by using a mobile illegally is dangerous and potentially very expensive. The only way of using a mobile legally whilst driving is to install a hands-free device, yet tens of thousands of motorists are taking unnecessary risks every year.

“Smartphones have further increased the temptation for drivers check email, reply to instant messages and access the Internet on the move - with potentially disastrous consequences.”

Halfords carried out the research to show the full extent of money wasted by not using a hands-free solution – such as a bluetooth enabled headset for  £10 or sunvisor mounted microphone and speaker costing from £45.

Half of the drivers who admitted illegal use of a hand-held phone said they had checked text messages, a third emails, one in eight had logged on to social network sites and a fifth had used an app – most commonly to check where they were going - the equivalent of 1.8 million motorists.

Almost eight out of ten drivers (78%) said they had spotted another driver using a hand-held mobile in the past month and six out of ten classed the practice as dangerous. It was the third most disliked bad motoring habit after drink driving and tailgating.

However there are signs that the increase in the fixed penalty fine - which increased from £60 to £100 last September – is helping force the message home.

Reports from individual police forces reveal that numbers caught are slowly falling. Seven years ago offences peaked at 165,000.