The BBC and ITV have put their main TV channels on the Internet for the first time - but they are only available to a small proportion of UK viewers.
BBC One, BBC Two and ITV1 are being broadcast 24 hours a day on the web as part of a low-key technical trial.
About 200,000 people in the UK can currently watch the simulcasts, which are testing a system of internet broadcasting known as multicasting.
It could pave the way for the whole country to watch live TV on the web. Around 4,000 tech-savvy viewers are being sought to report back on how well the multicasts work.
Until now, most broadcasts of video over the Internet have given each user their own individual feed.
Multicasting is closer to traditional broadcasting, with one high-quality signal sent out over the web for many people to tune into. But it is so far untested on a large scale.
Digital channels such as BBC Three, BBC News 24 and ITV2 are also available in the trial, as are the BBC's 10 national radio stations.
The BBC hopes it will be the first step towards allowing all UK broadband users watch its channels on their computers as easily as they do on TV.
But that is still "a long, long way away", according to Tom Loosemore, BBC head of strategic innovation.
"We want to offer live telly eventually and so this is clearly one area where we're trying to help the UK Internet become ready for that," he says.
"That's the overriding objective - to see whether multicasting is a route where the Internet can support live telly, because for a lot of people the Internet is a really major media environment now."
But he stresses the technology is still relatively unreliable and the trial is only open to a small number of viewers in the UK.
BBC channels are expected to be included in the corporation's proposed Internet TV service MyBBCPlayer, which is pencilled in to launch at the end of the year.
MyBBCPlayer will let viewers watch both live TV channels and the last week's programmes for catch-up viewing - all for free to UK broadband users.
A separate trial for the seven-day catch-up facility has just ended.
But MyBBCPlayer still has to be approved by the BBC governors, who will weigh up its public value. ITV, meanwhile, says the multicasting trial will help shape its strategy in the online world.
Simon Fell, ITV controller of emerging technologies, says: "It is clear that the general public is becoming more comfortable with accessing content online.
"We are constantly evaluating new ways to deliver ITV's well-known and compelling programming to our viewers, to maximise our revenues as a commercial broadcaster.
"This unique opportunity to trial multicast technology with Internet service providers and users will give us valuable insight into usage patterns and help to test this innovative and scalable solution to broadcasting over the Internet."
It is hoped the trial will encourage the online broadcasting industry to evolve and to persuade more internet service providers (ISPs) give access to multicasts.
Only eight ISPs are currently allowing users to watch the trial but, if successful, it is hoped that all UK ISPs will give access to the multicasts.
The trial comes as other major companies start selling TV shows and films on the Internet.
BT is planning to launch a service offering on-demand TV shows and films this autumn, while Sky now provides films to download.
In the US, Internet giants such as Google and Apple iTunes already sell TV episodes over the Internet.
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