Drivers caught using a handheld mobile phone at the wheel now not only face doubled fines and points penalties but may end up paying more than £100 a year extra for car insurance - if they are able to obtain cover at all.
As of March 1 motorists who are caught using their phone will be fined up to £200 - previously £100 - and receive six penalty points on their licence.
The conviction, known as a CU80 driving offence, will also significantly affect premiums - and may even lead to insurers refusing to cover the driver.
A 21-year-old administrator living in south-east London could normally expect to pay £701.31 a year to insure a Vauxhall Corsa, according to GoCompare, the comparison service. With a CU80 the premium would jump to £877.65 - 25pc more.
The cost of insurance generally reduces for older drivers because providers assume that experienced motorists are less of a risk.
However, those between 30 and 39 face a higher percentage price rise because this age group has the largest number of CU80 convictions, according to GoCompare.
For example, a 45-year-old driver, with the same details as above, would see their premium increase from £378.12 to £488.57 if they broke the mobile phone law - this is price rise of more than 29pc. A 35-year-old would see the biggest percentage premium increase with a CU80 conviction.
Why such large increases?
Insurers take a dim view of CU80s because there isn't really any excuse to use your mobile phone while driving - unless there is an emergency.
Providers consider speeding convictions, known as a SP30s, slightly differently.
Your premium is likely to increase if you are caught speeding but less so than with a CU80 because it can be done accidentally and you may have been in complete control of your vehicle while exceeding the limit.
As you can see in the chart below, the largest percentage increase is about 15pc with a speeding conviction - half the increase that may apply for mobile phone use while driving.
If you're prosecuted for using a handheld phone at the wheel, insurers may also assume that you do it often, making you higher risk.
Even if you're not caught by the police, telematics technology can allow insurers to track when you are using your phone while driving - and adjust your premiums accordingly.
Driving while using a handheld phone has been illegal since 2003.
Phone use isn't limited to making calls - you could be prosecuted if you check an app, social media or a map. This applies even if the car is not moving, such as at traffic lights.
You can use a handheld phone in the car in emergencies - such as if you need to call 999 and it is not safe to stop. Otherwise you need to be parked in a safe way.
It's also illegal to use a handheld phone when supervising a learner driver.
Motorists are allowed to use a hands-free device but you could still be prosecuted if it causes you to be unsafe on the road.
Have a question for our experts? Email firstname.lastname@example.org