Channel 4 has launched its internet-based on-demand TV service, 4oD.

Through 4oD, the broadcaster will make all of its commissioned TV content available for 30 days after broadcast, as well as around 1,000 hours of archived programmes stretching back 24 years.

The aim is to let viewers catch up with programmes or episodes of series they may have missed; the service also allows them to pre-book downloads of forthcoming shows so they do not miss them.

Each TV show will cost 99p to download and films will cost £1.99 and be available to watch as many times as the person wishes for two days; after this time the programmes or films, which are protected by Microsoft's digital rights management software, cannot be replayed unless paid for again.

Downloads vary in size but a 30-minute programme will be around 225MB, a one-hour programme about 450MB and a two-hour film around 900MB; download speeds will depend on the user's broadband connection.

Channel 4 is also looking at different subscription models that will let people download and burn programmes or films to DVD.

To use the service people have to download software that provides programme schedules and download listings. As a sweetener Channel 4 is letting people download and re-watch all the episodes of Celebrity Big Brother free of charge from tomorrow, January 3.

Touting it as the first on-demand service to be offered by a terrestrial television channel in the UK, a Channel 4 spokesperson, said: "4oD is about putting you, the viewer, in control and letting you choose what you want to watch, where you want to watch it.

"On-demand is the future of television - people don’t want to just be restricted to watching what they’re told to watch, they want to take control and run their own viewing experience.

"In its 24 years on air Channel 4 has broadcast some of the most loved, most challenging and most engaging programmes on British TV. 4oD lets you get at these whenever you want as well as helping you make sure you don’t miss any episodes of your favourite, current shows."