Allowing the SALT II limits on nuclear weapons to expire will encourage new Soviet and American weapons deployments that would shift the strategic balance to the advantage of the Soviet Union, according to a series of studies made inside and outside the government.

The Soviets' ongoing programs for building new weapons would allow them to field more new missiles more quickly than the United States, these studies conclude. Some Reagan administration officials who favor ending the SALT limits argue, however, that the Soviets are so far ahead of the United States that additional weapons they may build will have no measurable impact on U.S. security.

The administration is required by law to report to Congress by Saturday on its plans for complying with the limits in the SALT II treaty, which would have lapsed at the end of this year had it ever been legally ratified. Since 1981, both superpowers have said they would continue respecting the unratified treaty's main provisions.

The administration must also decide in practice whether to stay within the SALT limits this fall, when a new Trident submarine that would put this country over the limits will come into service.

Officials and experts on both sides of the issue acknowledge that the decision on how to deal with the SALT II limits will mark an important turning point in the history of the strategic arms competition. Many proponents of keeping the SALT limits say that allowing them to lapse would begin the total unraveling of the entire arms control system negotiated since the late 1960s, to the detriment of U.S. national security. Critics of the treaty say letting it lapse would force the Soviets to bargain seriously on new limitations.

One study by a Pentagon consultant showed that even if the United States responded by building 200 MX missiles (no more than 50 have been authorized by Congress), the Soviet advantage in overall destructive power and Moscow's ability to destroy "hardened" U.S. targets -- principally American missiles in their concrete silos -- would grow at an even faster rate.

The consultant also projected that today's arsenals, estimated at about 18,000 strategic warheads on both sides, could increase to 30,000 each by 1988.

In addition, several counting and verification provisions of the 1979 SALT II agreement would be lost, creating more problems for the United States in keeping track of Soviet nuclear forces, which increasingly will become mobile, according to defense experts.

On the other hand, some key present and former Reagan administration officials, including Assistant Secretary of Defense Richard N. Perle and former Arms Control and Disarmament Agency director Eugene V. Rostow, say that the SALT II limits have helped Moscow achieve strategic dominance, and that letting them lapse will create an incentive for Moscow to negotiate meaningful, deep reductions.

Both Rostow and Perle argue that Moscow is already so far ahead of the United States that an additional 1,000 or 2,000 warheads will not make a significant difference. Perle has also said that even if the SALT II limits remain in force, the Soviet lead over the United States will continue to grow, as it has since the beginning of the Reagan administration.

In a recent interview, Rostow argued that the United States should treat the expiration of SALT II limits as "part of the current negotiations" because Moscow wants to keep them in effect and "we haven't gotten a damn thing out of these past agreements."

Another top Reagan official observed recently, "If you don't have any limits , there can't be any cheating." He added that the administration's publicity about alleged Soviet treaty violations had led to "world condemnation," which to some degree was "an inhibitor on runaway weapons building."

This official cautioned, however, that "not everyone inside the administration agrees on that."

Supporters of the continuation of SALT II limits, whether within the administration, on Capitol Hill or among private interest groups, see the upcoming U.S. decision on the treaty as setting the direction of arms control for the foreseeable future.

"Sound diplomatic and military opinion recognize what the consequences will be of trashing the restraints embodied by the treaty limits," one Senate supporter of continuing the limits said recently. "They all know you need some limits in place while we look for more stringent ones."

The Federation of American Scientists has called it "strategic lunacy to let SALT II limits lapse if it could possibly be avoided," adding, "it is especially foolish to do it while threatening to build a defense against Soviet strategic weapons."

Although Washington and Moscow have pledged to avoid taking steps to undercut the weapons limits and operational understandings established by SALT II, each nation has put its own interpretation on its actions.

The Soviet Union, for example, had 2,504 strategic bombers and land- and sea-based intercontinental ballistic missiles (ICBMs) in 1981, when it made its statement. Moscow has never reduced to the 2,400 level that both sides would have been permitted had the treaty gone into effect. Nor have the Soviets dropped to the even lower level of 2,250 that both sides were to reach by the end of this year.

On the other hand, Moscow has destroyed some older land-based and submarine-based missiles to keep within the 2,504 figure. All the weapons destroyed were early models, much less effective than the newer ICBMs brought in to replace them.

For its part, the United States has destroyed some Polaris missile-launching submarines and old, land-based Titan missiles as newer U.S. strategic systems came into operation. Both older systems, however, involved single-warhead missiles that clearly were not considered effective under present U.S. nuclear strategy.

Washington may face a difficult decision this fall over whether to comply -- as it has in the past -- with another of the treaty's provisions when it puts into service the USS Alaska, a Trident submarine carrying 24 multi-warhead ICBMs. The Alaska will put the United States 14 missiles over the treaty limit of 1,200 for land- and sea-based, multi-warhead ICBMs.

An interagency group has been studying what can be done but, according to an administration official, "top-level" decisions have not been discussed with President Reagan.

Though a strong critic of SALT II before taking office, Reagan has not yet indicated what he plans to do about its limits. He has focused his recent comments on alleged Soviet violations of some of the treaty provisions.

Perle and other administration officials have privately suggested that instead of destroying a Poseidon submarine, whose 16 missiles, if dismantled, would bring the U.S. total below the treaty limit, the boat should be taken out of service, perhaps by placing it in drydock.

Such a "gray area" approach would not, they say, be an open breach of the treaty but would still allow negotiations to take place. They compare such a step to Soviet actions toward other treaty provisions that U.S. officials say violate the spirit if not the letter of the agreement.

One group that reportedly has not taken a position on the treaty limit extension is the Joint Chiefs of Staff. In 1979, the chiefs supported the SALT II treaty and its limits on the ground that the provisions put a ceiling, albeit a high one, on the growth of Soviet nuclear forces and thus permitted future U.S. planning to take place to counteract those forces.

Recently, individual members, such as the Army chief of staff, Gen. John A. Wickham, have said they favor continuing the limits. Gen. Bennie L. Davis, head of the Strategic Air Command, has said the same thing to congressional committees. Last year, Lt. Gen. James A. Abrahamson, director of the Strategic Defense Initiative Office, told Congress that negotiated limits restricting the number of Soviet offensive weapons would be needed if the "Star Wars" defensive systems were to have a chance of working.

Rostow dismissed the views of these generals, saying, "that was the same old argument the Joint Chiefs made all along."

Members of Congress, Pentagon analysts and some Reagan administration officials, however, recognize that the chiefs are concerned with present Soviet capabilities to build up their forces rapidly if the treaty lapses.

Pentagon officials, viewing the recent congressional vote to cut the president's Star Wars research and limit MX deployment, do not believe the American public is willing to support a new strategic arms race with the Soviets.

"One answer," an official said last week, "is to have some interim weapons restraints." He added, however, that "this administration is trapped by its own rhetoric" that condemned SALT II and thus now seems to require something new.