Drivers are ignoring pleas not to use hand-held mobiles, despite it becoming a specific offence since last December.
Driving while using a mobile can increase your risk of crashing fourfold
Anyone doing so risks a £30 fixed penalty, or a £1,000 fine plus three points on their licence if the case is disputed.
Police can also charge those involved in an accident with careless driving or dangerous driving, ultimately leading to a driving ban and fine decided by a court.
In January, breakdown rescue firm Green Flag found 90% of 700 drivers polled no longer used a mobile at the wheel.
But according to new research by RAC Legal Services, the number has crept back up, with more than a third of UK drivers - up to 10 million motorists - still using hand-held units while driving.
And as many as one in 10 surveyed in a poll of 2,000 motorists admitted texting each other while driving.
Mark Hodges, spokesman for RAC Legal Services, said: "This is something we suspected was happening, but our research proves the shocking number of drivers flouting the law.
"Drivers using a hand-held mobile phone pose a major threat to the safety of all road users and the law seems to have had little effect on their attitudes."
A spokesman for Green Flag said people had ignored the legislation because they "didn't feel they would get caught - they didn't think the police would stop them".
Green Flag is due to carry out more research by the end of the year and from anecdotal evidence expects there will "still be a hardcore of people" using hand-held mobile phones.
NUMBER OF FINES TO AUGUST
Greater Manchester - 3,155
West Midlands - 2,350
Strathclyde - 2,507
Simon Collister of the road safety charity Brake claimed far too few motorists were being stopped for the offence.
"Mobile phone use is absolutely prolific among drivers," he said.
"There have been random blitzes when huge numbers have been stopped but we know from reports that as few as four a month are being stopped by some constabularies."
He added: "Ideally, we would like the use of all phones while driving banned, including hands-free sets. It's no good having your hands on the wheel if your brain is elsewhere.
"In the meantime, we would like to see a much higher fine and a big prime-time advertising campaign."
Up to August, Strathclyde police had issued 2,507 fixed penalty notices, and West Midlands 2,350, while Greater Manchester Police had managed 3,155, with in excess of 5,000 offences detected. Northumbria police had issued 459 notices up to June.
Inspector Brian Wood, road safety officer for the Greater Manchester force, said: "We did a lot of raising awareness and officers were encouraged to target the new offence.
"Inattention in its various forms is one of the major contributory factors to road crashes. Government figures show you are four times as likely to be involved in a collision while using a mobile phone.
Hands-free sets are legal but not encouraged by road safety officers
"It splits the driver's attention and we have many anecdotes from the public backing this up.
"In a recent case, a member of the public reported an incident in which a bus driver went through a red light while using a mobile phone. This could have had horrific consequences.
"The driver is now being prosecuted."
Insp Wood also related incidences of people ploughing into roundabouts after driving down dual carriageways while on a mobile, texting while driving and looking away to scribble down addresses.
He added: "I would encourage all users of mobile phones to switch off while on the road and add four words to their voicemail message: 'I may be driving'.
"This shows you are responsible or, if you work for a company, that the company is responsible."
A Department for Transport spokesman said there was no excuse for not knowing about the law on driving and mobiles.
"We did a lot of campaigning to raise awareness when the law came in," he said.
"What we would advise is that all mobile phone use in cars should be avoided. That missed call won't kill you."
The Home Office said collated figures on the offence of using a mobile phone while driving would be available some time in the autumn.