The recently announced decision to sharply reduce the Soviet mission to the United Nations was made by the Reagan administration last September but was delayed for six months because of U.S. concerns that superpower relations would be disrupted, State Department officials said yesterday.

Details about the U.S. decision, which led to last Friday's announcement that the Soviets must reduce their diplomatic force in New York by more than 100 jobs in the next two years, emerged as the Soviet Foreign Ministry charged -- and the State Department denied -- that the forced cutback in Soviet officials at the U.N. could affect the prospects for another U.S.-Soviet summit meeting.

"This action has nothing to do with the summit meeting," State Department spokesman Charles Redman said. "There is no reason why this step should impair bilateral relations."

Redman's statement followed an assertion from Moscow that the cutback does "direct damage" to U.S.-Soviet relations and does not "create a favorable background" for another summit. The timing of a future summit meeting was in contention between the two governments even before the latest developments.

A State Department official called the Soviet protests "a measured response" which had been anticipated in Washington. He said Moscow's mention of the summit was unsurprising, because this is "the prism through which the Soviets see things these days."

The U.S. decision requires a reduction in personnel in the Soviet, Byelorussian and Ukrainian missions to the United Nations from the current 275 to a new limit of 170 by April 1, 1988.

White House and State Department sources said the decision grew out of escalating administration concern about Soviet and other East bloc spying in the United States. The arrest of John A. Walker Jr. and other members of his family on charges of espionage last May, as well as other well-publicized spy arrests, fueled the U.S. government's alarm, the sources said.

A meeting of the National Security Planning Group, a top-level committee of the National Security Council, decided last Aug. 7 that action should be taken to reduce the espionage threat posed by foreign operatives in the United States on grounds that it far surpassed the U.S. counterintelligence resources deployed against them.

The Aug. 7 meeting did not deal specifically with the Soviet mission to the United Nations, according to officials, but in subsequent weeks the State Department and other agencies identified the Soviets' large presence as a target for cutbacks. This was approved in high-level discussions early in September, sources said.

Sens. Patrick J. Leahy (D-Vt.) and William S. Cohen (R-Maine) had earlier obtained passage of a law requiring numerical equality between the number of Soviet embassy and consulate personnel here (320) and corresponding U.S. diplomatic personnel in the Soviet Union (270). But with U.S. personnel expected to increase as Americans replace Soviet nationals in embassy support jobs in the Soviet Union, a lowered ceiling for embassy personnel on both sides was seen to be self-defeating.

The Soviet presence in New York, including 275 at its U.N. missions and another 250 Soviet nationals employed by the United Nations secretariat, was described as more of an espionage threat than Soviet diplomats in Washington. The Soviet U.N. mission, moreover, could be cut without the expectation of an easy reciprocal action by Moscow.

Once the basic decision was made, one official said, the administration watched for the "least bad" time to implement it. Soviet Foreign Minister Eduard Shevardnadze's visit to Washington Sept. 29, Secretary of State George P. Shultz's trip to Moscow Nov. 4 and the Reagan-Gorbachev summit Nov. 19-21 all posed obstacles to announcement of the new policy, officials said.

Administration discussions early in January resulted in a decision to withhold the action until after the Soviet Communist Party Congress scheduled to begin in Moscow late in February, in order to avoid provoking a "fairly violent" reaction by Gorbachev in that forum, one official said. Last Friday, the day after the party congress ended, U.S. diplomats informed the Soviets and the United Nations of the cutback order.