Billed as the biggest change in the way viewers watch television in 40 years, the BBC has launched an online service that allows people to download many programmes from the last week.
BBC Director General Mark Thompson says the arrival of the "on-demand" iPlayer is as important as the first colour broadcasts in the 1960s.
Viewers can choose from 400 hours of programmes, between 60 and 70% of the total TV output, including hit shows such as EastEnders, Doctor Who and Planet Earth.
It faces competition from similar services provided by Channel 4 and ITV and from increasingly popular video-sharing sites such as YouTube.
The growth of the internet, mobiles and hard-drive recorders that save hours of programmes, has destroyed the notion of fixed TV schedules delivered through a TV in the corner of the room.
Broadcasters are under pressure to hold on to viewers by letting them watch programmes when and where they want.
"Our vision is for BBC iPlayer to become a universal service available not just over the internet, but also on cable and other TV platforms, and eventually on mobiles and smart handheld devices," said the BBC's Ashley Highfield, director of future media and technology.
The service, at www.bbc.co.uk/iplayer, is free, but people will not be allowed to save permanent copies to their computer. It could take 30 minutes to download an hour-long show.
It is only available to people living in Britain with computers that run the Microsoft XP operating system.
Programmes will be automatically deleted after viewing or after 30 days. Copyright protection software will prevent the copying of shows.
