Motorists who use hand-held mobile phones while driving will now face tougher penalties.
The Department for Transport says 21% of drivers break the law
The maximum fixed penalty fine has doubled to £60, and three points can be added to offenders' licences.
Motorists will also be prosecuted for using a hands-free phone if they are not in control of their vehicle.
Transport minister Dr Stephen Ladyman said those who flouted the law were "selfishly" endangering others, but a drivers' group criticised the move.
In 2005, 13 deaths and 400 injuries were blamed on drivers using hand-held mobile phones.
The Department for Transport says 21% of drivers admit breaking the law, introduced in December 2003.
Road safety minister Dr Ladyman said the new penalties would be combined with a "hard-hitting" television campaign.
He told GMTV: "We have been working with the constabularies around the country, we chose the timing of today with them, so that they are ready to help us enforce it."
He added it was just as dangerous for people to use mobiles while "wobbling" around roundabouts as it was on motorways.
A recent survey commissioned by Direct Line suggested one million people in the UK were flouting the law at any one time, with drivers in Cardiff, Newcastle and Southampton the worst offenders.
Inspector Douglas Kirkham, from Lothian and Borders Road Policing Branch, warned drivers could be prosecuted if they were using a hands-free or Bluetooth kit.
He said: "If while making or having a conversation, even if you're using a Bluetooth, you are not in proper control of your vehicle, then an offence has still been committed."
AA public affairs head of road safety Andrew Howard said: "Police can trace back on phone call records to establish use during a journey that ended in a crash."
Dr Vivienne Nathanson, head of science and ethics at the British Medical Association (BMA), said a BMA report in 2001 had warned of the dangers of mobile use.
"Drivers must get into the habit of switching off before they set off on their journey.
"There is also a responsibility on employers not to call their staff if they know they are driving and not to pressurise their staff to keep their mobiles on."
Jools Townsend, head of education at road safety charity Brake, welcomed the new penalties but said hands-free phones should also be banned.
Nigel Humphries, from the Association of British Drivers, argued those who could use a phone in a responsible way should be free to do so.
He said: "There are quite a lot of people out there who are perfectly capable of holding a conversation on a mobile phone while the driving comes first."
The maximum fine rises to £1,000 if the police or the driver choose to take a case to court rather than use a fixed-penalty notice, rising to £2,500 for drivers of vans, lorries, buses and coaches.