Defense Secretary Caspar W. Weinberger said yesterday that President Reagan will not give up development of his "Star Wars" missile defense plan in return for reductions in Soviet nuclear weaponry.

"The president has said that he will not give up the Strategic Defense Initiative or the opportunity to develop it," Weinberger said on ABC's "This Week with David Brinkley." "It's the only thing that offers any real hope to the world and we will not give that up."

National security affairs adviser Robert C. McFarlane, in a separate appearance on CBS's "Face the Nation," also dismissed the idea of trading with Moscow on Star Wars. "We think that is not the way to go," McFarlane said.

Both Weinberger and McFarlane, however, suggested that the United States will attempt to use the space initiative to win Soviet agreements on the use of systems to defend against -- rather than launch -- nuclear wars.

"We will discuss it in the context of offensive and defensive systems," Weinberger said. "And the Soviets have a great many defensive systems themselves."

The two appeared to be saying that although Star Wars will not be subject to any negotiations on offensive weapons, it might play into bargaining sessions on defensive weapons.

Soviet advances in defensive weaponry, including an extensive air defense system against bombers and an antiballistic missile program with a system in place around Moscow, are viewed by many in the administration as a potential threat to the arms balance.

But McFarlane said yesterday that such systems "can be a help to deterrence," suggesting that the administration intends to promote Star Wars at the negotiating table as a method of promoting world peace.

"Can both of us not be better off if we lower the level of offensive systems and integrate defensive ones?" McFarlane said. "We do recognize that you've surely got to talk about that, explain how and why we would be better off with defensive systems, but we're ready to do that."

Their comments come amid increasing speculation -- fed in large part by the administration itself -- that President Reagan might agree to use the $26 billion space research program as a bargaining chip in next month's arms talks between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei A. Gromyko.

The speculation involves a futuristic space-based defense system that is not yet off the ground. As envisioned by Reagan and Pentagon officials, the system would be capable of neutralizing any nuclear attack on the United States by destroying missiles at any point between their launch and their descent to Earth.

Construction of such a system is years away; some scientists are skeptical that it can be built at all. But the breadth of the concept -- and the implications if it works -- have aroused international interest and made Star Wars a key element in current arms negotiations.

In an interview last week, Weinberger said that Reagan had "specifically not excluded anything" from the Geneva discussions. A day ater, a senior administration official told reporters in a background briefing that space defense issues "have to be on the table . . . and they surely will be."

Reagan, in an Election Day interview with The Washington Post, said he regarded the Star Wars program as perhaps "the greatest inducement to arms reduction."

In recent days, however, the administration appears to have hardened its position on Star Wars, fearing that talk of possible tradeoffs will be interpreted by the Soviets as a lack of western resolve.

The administration's concerns were fueled last week by pointed criticisms from French President Francois Mitterrand and British Prime Minister Margaret Thatcher, neither of whom shares Reagan's enthusiasm for Star Wars.

After a weekend meeting with Reagan at Camp David, Thatcher pledged her support for research into defensive space weapons, but not necessarily for deployment. Deployment, she said, would have to be negotiated with the Soviet Union.

The Soviets have signaled their position. In London last week, Soviet Politburo member Mikhail S. Gorbachev called on the United States to scrap work on Star Wars and antisatellite weapons, contending that the U.S. initiative in space would kill any chances of reducing current nuclear armaments.

Yesterday, McFarlane dismissed Gorbachev's statement as "a little disingenuous. It implies that the Soviet Union surely has no interest in defensive systems, and that's manifestly untrue."

Both McFarlane and Weinberger contended that a Soviet buildup in defensive systems underscores the United States' need for a defense system of its own.

Star Wars "is the one thing the Soviets seemed to be determined to block," said Weinberger, who emphasized that Reagan will not bargain away the concept of a space-based missile defense system or agree to any limits on research.

"I don't see any way in the world there can be limits on research," he said. "In the first place it can't be verified. In the second place, it's an extremely undesirable thing to agree to."

Weinberger also dismissed as "highly speculative" a report that the administration had scaled back its goals for the space program. The New York Times yesterday quoted several administration officials, including White House science adviser George A. Keyworth II, as saying that the program would be aimed at defending the nation's land-based nuclear arsenal, rather than providing a protective shield for the entire nation.

When he proposed the space program in 1983, Reagan described it as an all-encompassing shield that would protect all Americans and make nuclear weapons "obsolete."

Weinberger said that an impenetrable shield remains the administration's goal and that Keyworth may have been referring to "a possible stage that may be reached sooner than another."

Keyworth made similar statements as long ago as June, however, and Defense Undersecretary Fred C. Ikle told reporters in October that the administration might opt for an "interim" defense system that would protect U.S. nuclear missiles but not cities.