The BBC launched its online television service on Friday, which is being called the biggest change to the way its viewers watch its programming in four decades.

The advent of content on the Internet is as big for the BBC as its first broadcasts in color in the 1960s, according to Director General Mark Thompson.

"This is a significant moment, as it heralds a new era when viewers will have the freedom to watch programs from the BBC's linear TV channels when they want," BBC Vision director Jana Bennett said. The application is in beta, and will be launched in the fall of this year.

The service, called iPlayer, does not permit users to permanently save programs. Instead, after viewing or 30 days, whichever comes first, the programs are automatically deleted. The software prevents any kind of copying.

iPlayer is only available to those living in the UK, and running Windows XP. The BBC said it takes about 30 minutes to download an hour-long show. Programs on the service are from the previous week's programming.

Although not available initially, the network does plan to eventually make available versions compatible with Vista and Macintosh. Other distribution agreements are in the works with MSN, telegraph.co.uk, AOL, Tiscali, Yahoo!, MySpace, Blinkx and Bebo.

It was not immediately clear whether those agreements would also be geographically restricted or open to a worldwide audience.