The Reagan administration has tentatively decided to disarm -- but not immediately dismantle -- two Poseidon submarines next month in a move that eventually could put the United States in technical violation of the unratified strategic arms limitation treaty (SALT II) for the first time, sources said yesterday.

The recommendation, which was discussed at a White House meeting Wednesday but has not yet received President Reagan's approval, is a response to administration allegations that the Soviets have violated the arms control agreement.

The plan comes a month before a new Trident submarine, the USS Nevada, begins sea trials. The new Trident, with its 24 intercontinental ballistic missiles (ICBMs), will push the United States over the SALT II limits unless two older Poseidon submarines, each carrying 16 missiles, are dismantled under the terms of the treaty.

The proposal calls instead for putting the two Poseidons, the USS Nathan Hale and the USS Nathanael Greene, in "caretaker status" by removing their nuclear missiles and placing the subs in drydock for five months to extract the nuclear fuel from their reactors.

Later, according to the plan, the unarmed and unmanned subs would be returned to the water and tied up at a pier for up to a year with their missile hatches closed, sources said.

Keeping the missile hatches closed and not beginning to remove the 32 launching tubes on the two Poseidons would constitute a technical violation of the SALT agreement, which requires that the hatches be open and the missile tubes destroyed after the Nevada has completed its sea trials later this year.

To compensate for that violation, one source said, the administration is considering an invitation to the Soviets to visit the subs for an onsite inspection to verify that the two subs are unusable. Yesterday, at a Senate Armed Services subcommittee hearing, Sen. John W. Warner (R-Va.) said Navy Secretary John F. Lehman Jr. had told him that such an invitation would be part of the caretaker plan.

At any time during the caretaker process, depending on Soviet behavior, the administration could order dismantlement in full compliance with the SALT agreement, sources said. On the other hand, if Soviet violations increased, the subs could be returned to service, but only after a two-year overhaul costing $170 million per boat, according to Navy sources.

Reagan has not yet finally adopted "caretaker status," but sources said he likes it because it contains elements to satisfy his top arms control advisers, who have been split for more than a year on how to respond to administration charges of Soviet treaty violations. When the last Trident submarine went to sea last fall, Reagan chose "to go the extra mile" and remain in compliance with the treaty by dismantling a Poseidon submarine, the USS Sam Rayburn.

The caretaker approach, at $17 million per sub, costs far less than overhaul and even less than dismantling, which Navy officials told Congress yesterday would require $20 million a sub.

At Wednesday's national security policy group meeting, the president decided to send Ambassadors Paul H. Nitze and Edward L. Rowny to consult with U.S. allies in Western Europe and Asia next week and announce his decision before he leaves Friday for the economic summit in Japan.

Capitol Hill sources said yesterday they had been told by British diplomats that British Prime Minister Margaret Thatcher will repeat her previous opposition to any U.S. plan to exceed the SALT II limits.

A SALT II opponent on Capitol Hill said yesterday he had been told by the White House staff that presidential adoption of the caretaker plan would not signal his final determination to break the treaty's limits.

This source said removing the two Poseidon subs from active service without fully complying with the missile launcher dismantling provisions would mirror recent Soviet action with regard to 70 SS11 ICBMs, which the Reagan administration had called a probable treaty violation.

The Soviet missiles were removed from their silos, as newer SS25 ICBMs became operational. The SS11 silos, however, have not been destroyed as required by agreed SALT procedures, he said, and could be rebuilt to hold new missiles.

Yesterday, House Armed Services Committee Chairman Les Aspin (D-Wis.), who supports continued compliance, questioned the sensibility of the caretaker approach. He argued that the Soviets could just as easily take submarines and missiles out of service, but return them to operational status much more quickly than the United States.

At a hearing with Navy officials yesterday, Sen. Carl M. Levin (D-Mich.) brought out the comparable costs of the various Poseidon programs. Levin said he favored a "proportional response" to Soviet violations, but added that "we shouldn't be responding in an illogical way."

One of the Poseidon subs now scheduled to go into the caretaker status was damaged last month while on patrol in the Irish Sea. On March 13, the Nathanael Greene "grounded while submerged," a Navy spokesman said yesterday, but was able to return to its base at Holy Loch, Scotland.

There were no injuries and damage to the sub was limited to its external ballast tanks and the rudder, the spokesman said.

The Nathanael Greene, which can still travel submerged, is on its way to Groton, Conn., the home port of its crew, sources said. Damage to the sub has been estimated at $2 million, according to congressional sources.