Time Inc. today announced it will begin publishing a weekly cable television magazine early next year.

Called TV-Cable Week, the magazine will be sold by subscription for about 70 cents an issue and will be marketed jointly with participating cable systems. Time says it could spend $100 million on the publication over the next five years.

Time executives, in announcing the magazine, were quick to play down reports that the new magazine would challenge the dominance of TV Guide, which sells 17 million copies weekly at 50 cents apiece.

"We don't think it will be head-to-head competition," said Kelso F. Sutton, Time group vice president for magazines. Later, Sutton allowed, "It perhaps would be somewhat redundant to have both magazines in the same household."

TV Guide senior editor Merrill Panitt said of the new publication, "We've always published for the viewers, not as a marketing tool. They have a twofold purpose, and one is to be a marketing tool for the operators."

The new publication's initial circulation will depend on how many cable operators decide to participate in the joint-marketing effort.

An estimated 4,560 cable systems now serve 26 million subscribers. Time Inc. itself is a major force in cable broadcasting. Its Home Box Office (HBO) is the largest pay-TV programming service, and its American Television and Communications Corp. (ATC) is the largest cable television system operating company. Another Time Inc. pay service launched in 1980, Cinemax, has been coming on strong.

Sutton said the ATC subsidiary is "not an automatic sign-up" as a participant in the new venture, which is scheduled to commence publication in the first quarter of next year.

The listing of local cable programs -- along with conventional broadcast fare -- will take up 64 pages of the magazine. Time Inc. says it has designed an elaborate computer system to process the massive listings data from subscribing cable systems throughout the country. The custom-designed program listings will be slipped into a 32-page national Time magazine-sized color feature publication.

Sutton said Time's market research shows that viewers are confused by the variety of information about cable programming that comes from magazines, newspapers and cable companies.

He said he anticipates an initial press run of several hundred thousand. "Five years out, it could be in the multimillion range," said Sutton. Based on TV growth projections, Time Inc. says TV-Cable Week could become the company's largest publication within a decade.