CBS's Showtime is developing an online video service for subscribers, according to a person with knowledge of the plans, joining rival cable channels that are seeking to reach customers away from TV sets. The service would be similar to the Web access being tested by Time Warner's HBO, said the person, who asked not to be identified because the plans aren't public. Showtime, with 18 million pay-TV subscribers and original shows including 'Nurse Jackie' and 'Weeds,' hasn't set a starting date. Premium cable channels are experimenting with ways to offer service online and to mobile customers without cannibalizing monthly pay-TV subscription fees that contribute the bulk of their revenue. Showtime is discussing the planned service with pay-TV operators that distribute the channel, the person said. 'There's nothing to announce at this time,' Johanna Fuentes, a spokeswoman for Showtime in New York, said in an interview. HBO, which along with Cinemax has about 40 million subscribers, in February began offering HBO GO at no additional cost to customers on Verizon Communications's FiOS TV service. 'All the HBO subscribers in the United States are going to have HBO programming on demand across every device,' Jeff Bewkes, Time Warner's chief executive officer, said on a May 5 conference call. 'That is a powerful offering.' Starz, owned by Liberty Media Corp., provides shows to Netflix, which offers the cable channel's content online as part of its monthly movie-rental subscription plans. Viacom's Epix, the premium movie channel first offered in October 2009, includes online access for all customers. Two- thirds have viewed movies online, with an average age of 36, according to a survey released today by Epix. Epix, also owNed by Lions Gate Entertainment Corp. and Metro-Goldwyn-Mayer Inc., has distribution with pay-TV systems including Dish Network Corp., Verizon's FiOS, Cox Communications and Charter Communications Inc. 