Time Warner Inc pay TV channel HBO will launch a service on Tuesday to let subscribers download movies and television shows over the Web, joining a crowded market for video on the Internet.

The service, HBO on Broadband, will make its debut with a much larger library than its cable-based on-demand service in a stab at satisfying increasingly mobile and choosy viewers.

It will roll out to Time Warner Cable high-speed Internet customers in Green Bay and Milwaukee, Wisconsin, as a free add-on to HBO and HBO on Demand, HBO spokesman Jeff Cusson said.

The company has no timeline for when it will be available to all U.S. subscribers, Cusson said.

HBO is entering movie downloads as older services that sell movies on the Web are reassessing their business models.

Apple Inc this month added online movie rental to its iTunes store to bolster movie sales, while Wal-Mart Stores Inc, which launched a movie download site in 2007 to great fanfare, quietly shut it last month for lack of sales.

But DVD rental company Netflix Inc offers customers unlimited viewing of movies online as part of its subscriptions, in a move to keep customers loyal.

HBO has positioned its site as a way to keep subscribers who watch more content online, just as it gave more flexibility to time-shifting viewers with the creation in 2000 of HBO on Demand.

Cusson said the roll-out of HBO on Demand reduced the cancellation rate among certain subscribers. They found more satisfaction with their service so they kept it longer, he said. As more viewing occurs on the broadband platform we want to make sure we provide that option to the HBO subscription.

Targeted at younger subscribers and travelers who watch TV shows and movies on laptops, HBO on Broadband will offer 600 titles each month, with 400 of those available at any time, as well as a live stream of the main HBO channel.

HBO on Broadband will also suggest titles based on viewing habits. Programming will be available for at least a month, but will be erased from users' libraries at its expiration date.

The application can be programmed for up to five users and downloaded to five devices that use Microsoft XP or newer operating systems. An Apple version is in the works.

Each month, an HBO original series such as Entourage, The Sopranos or Sex In the City will be available in its entirety.

(Editing by Braden Reddall)