More than a dozen TV networks - including one broadcaster, CBS Corp. (CBS) - have agreed to join Comcast Corp.'s (CMCSA, CMCSK) upcoming nationwide test of an online video subscription offering.

The agreements signal that a critical mass of major players in the TV business are lining up behind a concept championed by Time Warner Inc. (TWX) Chief Executive Jeff Bewkes known as "TV Everywhere," which aims to preserve the industry's lucrative subscription business model amid the rise of online video.

"Today's announcement highlights the industry's growing interest to bring long-form content to consumers via a secure and easy-to-use online platform," said Matt Bond, executive vice president of content acquisition with Comcast.

In addition to CBS, the other networks include cable network AMC, owned by Cablevision Systems Corp. (CVC); the Food Network, owned by Scripps Networks Interactive Inc. (SNI); and BBC America, owned by the BBC. Along with others, they joined Time Warner's HBO and Turner networks as well as Liberty Media Corp.'s (LMDIA) Starz.

Comcast, in a plan it calls "On Demand Online," will offer a Web authentication system that will allow 5,000 Comcast customers in markets across the country to verify their cable TV subscription in order to access programming online in an on-demand format at Fancast.com and Comcast.net.

The trial will provide online access for Comcast's TV subscribers to some of TV's most popular programming, including HBO shows like "Entourage" and "True Blood" and TNT's "The Closer." AMC's "Mad Men" and "Breaking Bad" are expected to be included later this summer, and the offering will also include major Hollywood films like "Dark Night" and "Juno."

Participation from a broadcaster like CBS is particularly noteworthy, as broadcasters have been putting popular TV content online for free on ad-supported sites like Hulu. Unlike their cable counterparts, broadcast networks depend mostly on ad revenue and have little incentive to limit access to their content in order to help pay-TV distributors preserve their business.

For its part, CBS has made much of its programming available at its own video site, TV.com, but the company has made progress recently in growing carriage fees that it gets from distributors to complement its ad revenue. Its participation in the trial signals the growing importance that carriage fees have to broadcast TV as ad markets suffer through the recession and Web players like Google Inc. (GOOG) steal market share.

A source at CBS said the company will offer programming that was not previously available online through the test, but its premium cable network, Showtime, will not be available.

Other broadcast giants like News Corp. (NWS, NWSA), Walt Disney Co. (DIS) and NBC Universal have yet to participate, although A&E Television Networks, which is participating, is part-owned by Disney and NBC Universal.Comcast, however, is still holding talks with most TV companies in an effort to bring more into the fold.

Seeing a potential threat from the rise of online video viewing, which remains in its infancy, Comcast and Time Warner recently agreed on a framework for putting programming from cable networks online to satisfy consumer demand while still requiring customers to subscribe to a TV service in order to prevent an erosion in their revenue base. Bewkes has said he envisions an industrywide authentication platform that will allow subscribers to any pay-TV service to access programming online on PCs and mobile devices.

-By Nat Worden, Dow Jones Newswires; (212) 416-2472; nat.worden@dowjones.com