YouView survey finds catch-up and on-demand services like BBC iPlayer that are streamed over a broadband connection now account for one-fifth of the nation's TV viewing.
Around 20% of television watched in Britain is viewed via catch-up and on-demand platforms, according to research by YouView.
Services like BBC iPlayer, ITV Player, 4oD and Demand 5, which allow programmes to be streamed after broadcast using an internet connection, account for six hours of TV viewing in the average household per week, a survey of 2,000 adults found.
While three-quarters of those surveyed still use their main TV set for most of their telly, a typical home has on average four other devices - such as smartphones, tablets and PCs - that can be used to watch TV programmes.
One in four also said they watch more TV now than they would have five years ago.
Steve Conway, head of marketing at YouView, said: "Television is a huge part of British life, but we know the way people view it is changing beyond all recognition.
"What is becoming important to TV fans is being able to watch whatever they want, whenever they want it, and this research supports that."
YouView is a non-subscription TV service that combines free-to-air channels, as available via Freeview, with a range of online features such as catch-up TV and on-demand players delivered via a broadband connection.
It is available as a standalone set-top box from retailers such as Amazon, Argos and John Lewis or with broadband packages from BT and TalkTalk.
Last week, YouView announced it would soon be expanding its on-demand TV offering to include seven-day catch-up for UKTV channels Dave, Really and Yesterday.