The Reagan administration is informing U.S. allies and members of Congress that it will begin dismantling two Poseidon nuclear submarines next month to stay within SALT II treaty limits, but is prepared to exceed the limits later if that is militarily beneficial, administration officials said yesterday.

The officials, who asked not to be quoted by name, said that the president's decision on the hotly contested issue is being taken to allied leaders by ambassadors Paul H. Nitze and Edward L. Rowny.

Reagan's "tentative thinking," made after a National Security Council session on the subject last Wednesday, will not become final until consultations with allies and members of Congress are complete, White House spokesman Larry Speakes said.

Speakes declined to disclose the substance of Reagan's ideas.

The decision to dismantle two Poseidons as a new Trident nuclear submarine goes to sea next month was described by other officials as dictated, in part, by military considerations.

To keep the two vessels as missile launchers, they would need to undergo a two-year, $340 million overhaul.

Reagan is said to have decided against a plan to put the two older submarines on "caretaker status:" disarmed, but not dismantled. The Washington Post reported Saturday that the administration tentatively had decided on this option.

Such a move which would have been a violation of SALT II, had been favored by Defense Secretary Caspar W. Weinberger but would not have provided an increase in U.S. capabilities, an administration source said yesterday.

"The path is set when it makes sense militarily for the United States to exceed the limits under SALT II, unless the Soviets cease their pattern of violations," said an official familiar with Reagan's decisions.

This official suggested that the SALT II limits could be exceeded as early as this summer, when the Air Force nears the treaty's limit on the number of bombers carrying air-launched cruise missiles.

"The direction is clear," said the official, who has favored exceeding the limits as a response to Soviet violations.

Other U.S. responses to violations may include increases in military programs not limited by SALT II, including sea-launched cruise missiles of an advanced type, officials said.

Speakes, in a written statement, said: "What we do in the future depends on our national security needs, and our commitments to our allies, in meeting the threat that we face, which in turn depends on what the Soviets do."

The dismantling decision is being forced because the Trident submarine USS Nevada is to begin sea trials next month. Its 24 multiwarhead missiles will put the United States 22 misssiles above the SALT II limit for such weapons.

To adhere to the treaty, the United States must dismantle an equal number of missiles and their launchers. The two Poseidons, each of which carries 16 multiwarhead missiles, satisfy that requirement.

In a related area, the Department of Energy said yesterday that another underground nuclear test will take place at the Nevada Test Site today. Code-named Jefferson, it will be the third announced U.S. test this year and the 10th since last July, when Soviet leader Mikhail Gorbachev announced a moratorium on his country's testing.

Reagan, who rejected Gorbachev's request that he join the moratorium, had invited Gorbachev to send scientists to observe this test and a new on-site monitoring device that records the explosion's force.

Gorbachev rejected the proposal, saying that it would be like "asking a man advocating the abolition of capital punishment to witness an execution."