John Chancellor has confirmed that he plans to leave his job as anchorman of NBC Nightly News to become commentator and roving reporter for the network.

Both he and NBC News president Lester Crystal yesterday emphasized that no date has been set for the change. "All I knew," Chancellor said yesterday "is that it definitely is not soon."

Crystal, responding to rumors that have persisted since July, when Chancellor first signed a 10-year pact with the network and notified NBC that he eventually wanted to leave the anchor post, said in a statement yesterday.

"John Chancellor is not leaving his position as NBC Nightly News anchor. He is staying on. His talk to his staff (Thursday) was to explain his long-term arrangement with NBC NEws in which he will someday become a commentator and special reporter . . . no time has been set for this to happen."

Sources at NBC indicated that David Brinkley will continue as the Washington-based co-anchor of the evening news show when Chancellor makes the change in assignment.

Chancellor said yesterday that the persistence of the rumors in the New York press was the principal reason he revealed his plans Thursday.

"I finally got to the point," he said, "where I went to Les (Crystal) and said 'I can't take it any more' and he said 'you're right.'"

Those rumors have consistently linked Chancellor's departure with his replacement by Tom Brokaw, who currently hosts "The Today Show" on NBC.

In turn, there has been speculation that either "Tomorrow" host Tom Snyder, or Phil Donahue, whose syndicated show is increasingly popular, wouls succeed Brokaw on the early morning network program.

Sources at NBC yesterday confirmed that Brokaw is currently considered the frontrunner for the anchor spot when Chancellor takes his leave, but that Crystal has not had serious talks with Brokaw on the subject.

"It's much too early in the game for that" one executive said. "We honestly don't know when John wants to leave the anchor job."

The source indicated that while NBC is interested in Donahue, the Chicago-based talk show hose incorrectly inferred from a recent vistit by an NBC executive that he was being interviewed specially for the "Today" show. Donahue subsequently notified the mess he was under consideration to replace Brokaw.

The NBC executive added that while snyder recently rejected an offer to host the upcoming prime time magazine show for NBC the "Tomorrow" host was not out of the running for the "Today" job. Undoubtedly other names in and outside NBC will surface in the months ahead as possible replacements.

Chancellor yesterday described himself as "a very happy man. I've got more job security than anybody else at NBC." He said his contract guarantees "a comfortable wage" until his 60th birthday.

The joking reference to "job security" comes at a time when NBC's upper echelon of executives has been shaken by a series of recent dismissals, including that of Chancellor's own boss, Richard Wald, who was replaced late this summer by Crystal as head of NBC News.

A case could be made that the network's signing of Chancellor to a long-term contract in July was an attempt to assure an appearance of stability in the news operation even as low ratings continued to bedevil top NBC brass on the prime-time entertainment side of NBC.

Chancellor said that he has been talking to friends "for at least five years" about taking a commentator's job with the network, an idea, he said, that was prompted originally by the "really beautifully done" columns written for Newsweek during the Vietnam war by the late Steward Alsop.

"I thought then that some day I'd like to be able to do that kind of thing on TV," he said.

Chancellor said he never talked seriously to CBS but confirmed that he let NBC know last summer he was interested in possibly replacing Eric Sevareid, who retired this week as that network's resident commentator.

That interest, he said, led to the discussions with NBC that guaranteed his chance to move to a similar job whenever NBC finds a suitable replacement.

Chancellor said he and Walk discussed the move last spring and summer, Crystal, who replaced Wald in late summer, was kept informed of the discussions.

Chancellor said "When I stop being anchor, I'll appear on Nightly News three or four times a week as a commentator. There'll be some travel involved - my new contract permits me a month every year of foreign travel - and I'll occassionally do the longer 'Segment III' pieces."

"Segment III" is a new feature on the Nightly News that runs five minutes every night for in-depth treatment of major topics.

Chancellor said there has been "absolutely no pressure at NBC" over the fact that the Nightly News has, except for several brief periods, remained second to the CBS Evening News with Walter Cronkite over the past seven years.

Both CBS and NBC have enjoyed long ratings leads over ABC Evening News during that period.

"Of course, there's always a feeling you'd like to be No. 1," Chancellor said yesterday. "But you have to look at it this way: The loyalty of the audience can't be adjusted by an programming changes: people stick to programs pretty much.

"Look," said Chancellor, "I'm a very, very happy guy. And it has occurred to me that I'm the only person in TV who have ever quit one of these (anchor) jobs. I may be in the Guinness Book of Records one of these days."

Crystal, who was unavailable to reporters yesterday, issued a statement in which he attacked a New York report that he had called Chancellor in Israel during the recent Sadat-Begin meetings, to complain that NBC was being scooped by ABC and CBS.

"The report," said Crystal, "that I called my friend and colleague in Israel and 'called him on the carpet' is untrue and ridiculous."