It will be the "NBC Nightly News with John Chancellor" starting Monday evening, as the network yesterday relegated veteran David Brinkley to the role of a three-nights-a-week commentator on the faltering news program.

The sudden move returns Chancellor to the single anchor role he held on the program from 1970 until 1976.

During that period, Brinkley had conducted his "David Brinkley's Journal" commentary on the program, following the retirement of his longtime co-anchor, Chet Huntley.

"The Huntley-Brinkley Report" had been the number one news show in the country all during the 1960s.

Yesterday's change was the first visible major step taken by NBC News president William Small to revamp the Nightly News since he moved from CBS last month.

Reflecting a trend that first became noticeable in June, ABC World News Tonight has finished second to CBS Evening News in the three-network news ratings race in three of the past four weeks and has even begun to challenge CBS.

The decision to drop Brinkley as co-anchor followed Small's one-week visit to the NBC News bureau here in mid-September.

A network spokesman said yesterday that on Sept. 25, Brinkley had written a memo to Small, saying he wanted out.

However, a key NBC source told us yesterday "the memo from David was an artifice -- it was Small's decision all the way."

Brinkley would say yesterday only that he's been asked to "do three commentaries a week, some specials here and there and something to do with the conventions . . . and that's all I know about it."

Small said yesterday that Brinkley will co-anchor the 1980 conventions with Chancellor, as well as other important elections, will report from around the country during the election year and "has been penciled in for at least one major White Paper" (the NBC documentary series).

Small insisted that the move "will beef up our political coverage in 1980."

He said, "David has strong feelings (apparently reflected in the Sept. 25 memo) about what's happening in this country, and he wants to take a first-hand look and report on it."

"I am a great Chancellor fan and a great Brinkley fan," Small said yesterday. "They're as strong a team as you will find."

Small had been a CBS News executive since 1962 until his surprise move to NBC in late summer.

With Huntley's retirement in 1970, the CBS Evening News with Walter Cronkite as a single anchor became the dominant news show, while NBC Nightly News continued to enjoy a comfortable second-place lead over ABC.

But in the past two years, with the naming of Roone Arledge as president of ABC News, the rise in that network's prime-time audience and a subsequent loss of NBC affiliates to the upstart network, NBC's position has been crumbling.

Brinkley has been with NBC since 1943 and began co-anchoring in a network TV news show for NBC with John Cameron Swayze in 1956.