The Reagan administration denied yesterday as "utterly without foundation" a report in The Washington Post that the United States is prepared to negotiate with the Soviet Union any proposed deployment of new strategic defensive weapons and would give Moscow five to seven years' notice before unilaterally deploying them.

White House spokesman Larry Speakes said the report in Wednesday's Post was "made up out of thin air," and added that there "was obviously misunderstanding." State Department spokesman Charles Redman said if the information had been "thoroughly checked, it would have been shown to be utterly without foundation."

At the Defense Department, a senior official who declined to be identified called reporters to his office to say that the idea outlined by The Post "has not been discussed, much less agreed to."

The Post report, attributed to Defense Department sources, said the proposals were secretly agreed upon on Oct. 4 by the Special Arms Control Policy Group (SAC-G), which maps U.S. strategy in the Geneva arms control negotiations.

The report quoted the sources as saying the new proposal is intended as an amendment to the 1972 Antiballistic Missile (ABM) Treaty provision that permits either superpower to withdraw from the treaty on six months' notice. According to the Post article, the U.S. proposals would extend the notification period to five to seven years.

A source close to the SAC-G discussions told The Post that the secret Oct. 4 agreement stemmed in part from a desire of some U.S. officials to have tangible proposals for the Soviets to consider at arms control talks under way in Geneva. The U.S. role in the phase of talks focusing on space has thus far been restricted largely to arguing the intent of the ABM treaty and the need for the superpowers to consider the interrelationship of nuclear arsenals and strategic defenses, the source said.