The Reagan administration, reversing the legal interpretations of previous administrations and some of its own past statements, has decided that testing and development of exotic antiballistic missile systems such as those in the "Star Wars" program are permitted under the 1972 ABM treaty.

The administration's new interpretation of the treaty was confirmed yesterday by a senior White House official who briefed reporters on U.S. objections to the recent Soviet offer of a 50 percent cut in certain offensive missiles in return for a ban on Reagan's Strategic Defense Initiative, or Star Wars. The Soviet offer was described in the briefing as "a place to start" but in its present form one-sided and threatening to U.S. security.

White House national security affairs adviser Robert C. McFarlane volunteered a new interpretation of the 13-year-old Antiballistic Missile Treaty in a television program Sunday. Yesterday the senior White House official, who cannot be identified under the ground rules of the news briefing, confirmed that McFarlane's televised remarks reflected what is now the fixed policy of the administration.

Retired ambassador Gerard Smith, chief U.S. negotiator of the ABM treaty, said the administration's interpretation "makes a dead letter" of the treaty. Smith said he believes it would make possible almost unlimited testing and development under Star Wars, and probably also actual "building" of the space-based antimissile system "as long as you did not deploy."

Administration sources said a new interpretation of the treaty had been under discussion and, at times, intense debate since last summer within the administration's Senior Arms Control Group, or SAC-G.

The administration was moving in the direction indicated by McFarlane in recent weeks -- though not to the point of claiming the treaty "authorized and approved" the testing, which were the words McFarlane used Sunday. In administration discussions, sources said, the issue was whether the treaty could be interpreted as permitting such activities. A final decision was "not completely clear" even after McFarlane made his remarks on "Meet the Press," an official said.

One official said the still-secret negotiating record of the ABM treaty is "ambiguous" on the point in question and subject to "a well justified disagreement" within the government. However, this view is disputed by Smith and John Rhinelander, legal counsel to the U.S. delegation that negotiated the ABM treaty.

The nub of the issue is whether an "agreed statement D" between the U.S. and Soviet delegations at the time of the treaty signing on May 26, 1972, gives a broad exemption from the restrictions of the treaty for future types of ABM systems "based on other physical principles" such as lasers and directed-energy weapons. Many elements of the administration's Star Wars research program are based on such exotic technology.

The purpose of agreed statement D, it said, was "to insure fulfillment of the obligation not to deploy ABM systems and their components except as provided in Article III of the treaty," which originally allowed both countries to maintain two conventional ABM systems, based on antimissile missiles.

The agreed statement said that if new ABM systems "based on other physical principles" are created in the future, "specific limitations on such systems and their components would be subject to discussion . . . and agreement in accordance with Article XIV of the treaty" -- the article explaining how the treaty could be formally amended.

Until the administration's recent change of mind, that had been interpreted to mean that testing and development of exotic technologies were not legal, except possibly for new versions of fixed, land-based systems that the treaty allowed. Article V of the treaty formally precluded any testing or deployment of "ABM systems or components which are sea-based, air-based, space-based or mobile land-based."

Previously the administration's plans for tests of elements of Star Wars have been justified as complying with the ABM treaty on completely different grounds: that these projects were of such low quality, power or reliability that they did not qualify as "components" of an ABM system, or that they could be modified so as not to appear to be part of an illegal system.

Smith and Rhinelander said it was wrong to interpret the "agreed statement" as sanctioning testing of ABM systems or components that are flatly ruled out elsewhere in the treaty. "It is just impossible that an agreed statement supersedes a provision of the treaty," Smith said.

The administration's 1983, 1984 and 1985 Arms Control Impact Statements submitted to Congress by the Arms Control and Disarmament Agency took the position that the ABM treaty does put restrictions on ABM programs based on "directed energy technology" or other exotic technology "when such DE programs enter the field testing stage." The 1986 Arms Control Impact Statement, submitted this April, omitted that statement.

The senior official who confirmed the administration's current position said the Soviet Union had never accepted an interpretation of the treaty that banned "research, testing, development of systems based on other physical principles."

The official said there had been "unilateral statements" made that the treaty ought to limit such exotic systems but he added that "never have the Soviets bought that."

The proposed cutbacks in the new Soviet arms control offer are "inappropriately linked" to the demand that the United States stop its Star Wars program, the senior official told reporters yesterday. "It's a precondition that must be dropped," he said.

That the Soviets have made an offer of deep cuts is "a very good development," and a sign that Reagan's policies have paid off, the official said. U.S. negotiators will pursue the details in Geneva, he added.

Most of the White House presentation, though, was centered on objections to the Soviet proposal, especially inclusion of U.S. Euromissiles and "forward based systems" among the strategic weapons to be cut by half. This would produce "highly unequal" forces with great advantages to Moscow, the official said.

Those two categories, described as support for U.S. allies, were said to consume 1,149 of the U.S. entitlement of 1,680 strategic nuclear delivery systems under the Soviet plan. The United States would thus have only 531 missiles or bombers left for deterrence against Soviet nuclear attack, and these would be threatened by a much larger number of Soviet weapons.

The White House also said the Soviet proposal might unfairly hamper U.S. military modernization and could have serious verification problems. These aspects of the Soviet proposal have yet to be fully presented in Geneva, it said.