Research by media regulator Ofcom has shown that 13.5m Britons now use their phone to regularly access the internet, almost triple the number that did in 2008.
The regulator has also found that internet take-up in the UK has now reached 73%, the majority of which is through fixed broadband.
It shows the importance for motor retailers of embracing online marketing in order to reach prospective customers.
Its annual Communications Market Report says people spend seven hours a day watching TV, using the Internet and using their mobile phones.
Many young adults multi-task their media consumption, such as by watching TV while surfing the internet on their mobile.
Radio continues to be an important medium, with 91% of people able to access radio services.