The Shubert Organization, the nation's largest theater chain, has temporarily won the right to book shows into the National Theatre.

On Monday in New York City, U.S. District Court judge Morris E. Lasker modified a 24-year-old consent decree that restricts the giant Shubert group -- which owns about half of the large theaters on Broadway -- from acquiring or booking any new theaters. The decree also requires the organization to petition the court in order to expand its operations into new theaters.

Lasker's action permits the Shubert Organization to act as the National's booking agent through Aug. 31, 1981 -- replacing the Kennedy Center, which had booked the house until Jan. 10 of this year.

Lasker must still rule on whether the Shubert group can manage the National on a permanent basis.

The court action is the latest development in a heated rivalry between two of New York's theatrical giants. The Shubert move had been opposed by the group's stiffest competitor, the Nederlander organization -- which is second only to Shubert in its ownership of theaters and which is rumored to be interested in booking the National Theatre itself.

Nederlander filed a 72-page memorandum protesting the Shubert request to book the National. And, according to Nederlander attorney Jonathan Weisgall, the group has asked the Justice Department to investigate the Shubert Organization for possible anti-trust violations of the consent decree which has ruled them for 24 years.

Shubert and Nederlander have been fierce competitors for years. "There's no love lost between them," said one observer of this latest battle.

In the last five years, Nederlander similarly opposed a Shubert application to manage a theater in Los Angeles. The court allowed Shubert to do it.

Weisgall explained the anti-trust complaint by arguing that Washington -- along with Boston and Philadelphia -- is an extension of the New York market, since the cities all have similar audiences. As a result, they are essential both for touring and for try-outs.

"Try-outs are extremely important," said Weisgall, who noted that his studies showed 60 percent of the plays that opened on Broadway tried out on the road first. "The overwhelming number of try-outs were in Washington, Philadelphia and Boston. And Washington was the leader."

"Do the Shuberts own a lot of theaters in Washington? No," said Weisgall. "But it's a bigger issue than that. Washington is not in a vacuum."

Weisgall noted that the Shubert Organization owns a theater in Boston and another in Philadelphia as well, and argued that, "In legal terms, this is an extension of their New York power."

Shubert attorney Alvin Stein, contacted in New York, said "the Shuberts don't regard Washington as merely a try-out stop before New York. They regard it as an important city for important theater.

"Any contribution that the Shuberts could make to Washington is only constructive," Stein said, "and will only make theater better in Washington. The Shuberts' aim is to bring the very best in legitimate theater to Washington."

Gerald Schoenfeld, head of the Shubert Organization, yesterday said he had no comment on the conflicting opinions. As for the Nederlander complaints, Schoenfeld said: "That's what we have the courts and the Justice Department to decide."

Kennedy Center chairman Roger Stevens, who is not pleased that the National terminated the Center's booking arrangement with them, said he does not care who books the National. But Stevens argued that the Shubert Organization should bid on ownership of the National Theatre so that the ownership and booking would be in the same hands.

"I'm just suggesting that the Pennsylvania Avenue Development Corporation put it legitimately out for auction," said Stevens. (The Marriott and Quadrangle companies are in the process of buying the theater from PADC and are planning to renovate it.)

Stevens said, "I told Bernie Jacobs [one of the Shubert heads], 'If you don't come in and make an open bid, I'll be as troublesome as possible.'"

Maurice Tobin, president of the New National Theatre Corporation, said the Nederlander organization had submitted a proposal for managing and booking the National. "But they got eliminated by our search committee very quickly," said Tobin. (The committee considered some 27 agencies and individuals before choosing the Shuberts.)

"The Nederlanders and [Roger] Stevens were together on a lot of shows," Tobin said, and since the National had just dissolved its relationship with the Kennedy Center, it didn't want to start one with a Stevens collaborator. t

Shubert lawyer Alvin Stein said "the Shubert made a proposal to the National and Nederlander made a proposal to the National. The National concluded the Shubert made the most attractive proposal. The Nederlander reaction is that of a disappointed suitor."

But a source close to the Nederlander organization said "the point [of the current dispute] is not to get the Nederlander organization control," and that James Nederlander, head of the organization, had told the Justice Department attorneys that he preferred an independent agency coming in to run the National.