Some 36% of those accessing the internet do so with tablets, with ownership of the gadgets growing 63% year-on-year to 17.9 million Britons in February, according to the Internet Advertising Bureau UK (IAB) Digital Adspend report and online audience measurement company UKOM/ComScore.
Almost six in 10 tablet owners (57%) say it is their "go-to" device to surf the internet at home, while 66% say it is easier to go online using a tablet.
Almost two-thirds (65%) like to use them while watching TV.
Banking and finance is the area of people's lives that would be most affected without the internet or mobile phone, cited by almost half of Britons online (48%), followed by keeping up with current affairs (37%) and relationships with friends and family (35%).
The IAB report shows that overall advertising on the internet and mobile phones increased by 15.2% or £853 million to £6.3 billion last year.
Investment in mobile advertising grew by 93% to £1.03 billion last year, with mobile accounting for 16% of digital advertising spending, compared with 10% in 2012.
Social media advertising grew 71% to £588.4 million.
IAB director of research and strategy Tim Elkington said: "Digital advertising continues to grow at impressive rates simply because marketers are becoming more responsive and savvy to the increasing ways people consume content across different devices.
"However, there's still a lot of work for the industry to do when it comes to tablet advertising. Spend on ads designed specifically for tablets is growing fast but it's still a very small part of the pie, despite the increase in tablet ownership and the crucial role they play in people's internet use at home.
"The tablet has moved from the offices of early adopters to the nation's living rooms and advertisers should be following suit."