The Reagan administration yesterday made public its decision against the sale of new sophisticated warplanes to Taiwan, saying that "no military need for such aircraft exists."

The State Department announcement, confirming earlier news reports, came as Assistant Secretary of State John H. Holdridge conferred in Peking with the People's Republic of China on the contentious issue, which has threatened to cause a serious breach in Sino-American relations.

Informed sources said the U.S. decision flowed from a National Security Council meeting on the issue chaired by President Reagan last Thursday at which his principal advisers recommended the course that ultimately was followed. Holdridge and a team of U.S. experts was dispatched to Peking Saturday morning, and a few top congressional leaders were consulted late Saturday.

Reagan made the final decision at Camp David Sunday afternoon, the State Department said.

Sen. Jesse Helms (R-N.C.), a backer of Taiwan, said the decision is "difficult to understand, particularly in the light of Reagan's constant assurances in the past," and he predicted a reaction among conservatives.

Helms said Reagan volunteered to him only last month his "total support" for Taiwan. The Reagan-Helms conversation took place as the president persuaded the senator to lift his objections to a pending consular convention between the United States and China. "If we're going to play rhetorical games, I will have to be more cautious in the future," Helms said.

Sen. John Glenn (D-Ohio), who has been involved in discussion of the issue both in Peking and Taipei, said he did not disagree with the presidential decision but that he questioned the need to rush it through without thorough consultation with Congress. Glenn said Secretary of State Alexander M. Haig Jr. had "guaranteed" that lawmakers would not be bypassed but that, in his opinion, this pledge had not been kept.

Rep. Stephen J. Solarz (D-N.Y.), chairman of the House Foreign Affairs subcommittee on Asian affairs, approved Reagan's "farsighted decision" and added that "I know it was a difficult decision in view of his longstanding friendship with Taiwan." Solarz and other subcommittee members recommended against selling the sophisticated FX fighter plane that Taiwan had requested.

The White House had nothing to say, leaving it to the State Department to announce the politically delicate decision in a midday press statement.

The statement, read by spokesman Alan Romberg, said that since the beginning of his administration Reagan "has been conscious of the need to carry forward the unofficial, people-to-people relationship between the United States and Taiwan" and that he repeatedly expressed "his personal concern for the continued well-being of the people of Taiwan."

The statement repeated the commitment of the U.S. government, set forth in the Taiwan Relations Act of 1979, to provide such military equipment as may be necessary to enable Taiwan "to maintain a sufficient self-defense capability."

A study of Taiwan's defense needs over many months by U.S. agencies had led to steps toward making necessary "self-defense" sales, and more are anticipated, according to the statement.

The "conclusion" of the U.S. agencies, however, is that "no sale of advanced fighter aircraft to Taiwan is required because no military need for such aircraft exists." For the foreseeable future, Taiwan's defense needs can be met by replacing "aging aircraft" in its inventory and by extending the production line of the Northrop F5E, currently the island's mainstay fighter plane.

Administration sources said a plan to enhance the capability of the F5E had been examined but rejected for the foreseeable future. At one point this had been considered a compromise plan that could give some added capability to Taiwan and yet avoid the sale of a high-profile fighter plane such as the F5G or F16 that would bring about a crisis with Peking.

The decision of the Joint Chiefs of Staff to reverse its earlier support for sale of a more sophisticated fighter to Taiwan was described by one source as an important element in administration decision making. The military chiefs are reported to have been motivated in large part by concern that a falling out with China could weaken the common front against the Soviet Union and thus adversely affect the global military situation.

The initial reaction by Chinese leaders to the presentation of the administration decision did not give a clear reading of the immediate course of Sino-American relations, administration sources said.

Taiwan had nothing to say publicly about the decisions yesterday. While disappointed and unhappy, Taiwanese authorities may decide on a relatively restrained public response, sources said.