Leonard Bernstein will not conduct the National Symphony Orchestra next spring as scheduled. Instead, he will devote all of his time during 1980 to composition.

Bernstein advised National Symphony music director Mstislav Rostropovich last week that he will be unable to fulfill his engagement to conduct the final three weeks of the orchestra's subscription concerts in April and May.

Berstein's announcement partially confirms what has been rumored for some months. The lates issue of the Central Opera Service Bulletin stated last week that Bernstein was at work on an operatic version of Vladimir Nabokov's novel. "Lolita." That rumor, however, was followed by reports that the famous composer-conductor had already abandoned the project.

In his dual role of composer-conductor, Bernstein has several times taken similar leaves of absence from the responsibilities of the latter to fulfll the drives of the former. In 1958 when he was working on "West Side Story," which opened at the National Theater in 1958, and again in 1971 when the score of "Mass" was in the works for the opening of the Kennedy Center, the composer took precedence over the conductor.

Those close to the orchestra were taking the crisis philosophically. Oleg Lobanov, managing director said, "I'm sorry he's canceling. He's a marvelous person and a marvelous conductor. At the same time I am awfully gratefully to [substituting conductors] Erich Leinsdorf and Max Rudolf. They are both great pros.

"I know that Maestro Rostropovich and I share the view that Bernstein has, for his own creative reasons, decided he must take this time to compose. And I'm sure that something wonderful will come out of it."

NSO Board President Austin Kiplinger, confirming that Bernstein's composing project is indeed an opera, was equally positive. He said:

"Obviously I am very disappointed. But if you take it in the total sense, this is probably the last chance he has to buckle down and write that opera. In the arts, a man has to have the isolation and the time to place to get these things down on paper. I think Bernstein has this in his head and now he just has to have the time. He has divided his time into watertight compartments and next year has to be for composing.

"At the same time," Kiplinger added, "I am very glad that Max Rudolf is coming back. You remember that he came in before on a cancellation and gave us one of the best concerts of the season."

Faced with the loss of one of the world's biggest-name conductors, Rostropovich and Lobanov promptly arranged for performances of one of music's largest works, the Eight Symphony by Gustav Mahler. Often called the "Symphony of a Thousand," because of the huge numbers required to perform it, the Mahler Symphony will be conducted by Leinsdorf on April 29, 30, and May 1.

These will be the first Kennedy Center performances of the music which requires two large choruses, a children's chorus, eigth soloists and an enlarged orchestra plus organ. Leinssdorf's appearances in the spring will be in addition to his previously announced NSO concerts in October.

Max Rudolf, who replaced Bernstein last April when an attack of dysentery prevented him from closing the National Symphony season, will return again next May. On May 6, 7, 8 and 9 he will conduct with pianist Bruno Leonardo Gelber as soloist, and on May 15 and 16 with Irish flutist James Galway, who will be making his National Symphony debut.

Yesterday the NSO also announced that pianist Lili Kraus will be the soloist with the orchestra October 16 through 19, under guest conductor Kazuyoshi Akiyama, when she will paly the Concerto No. 9 by Mozart.