NATO defense ministers today unanimously urged research into President Reagan's space-based missile defense system, calling the project "prudent" in light of Soviet efforts in strategic defense.

Despite the first NATO nuclear planning group endorsement of Reagan's plan, however, European ministers at a two-day meeting here questioned the future impact of a U.S. space-based strategic defense should the project ever move beyond the study phase.

The NATO secretary general, Lord Carrington, said at a news conference that European participants shelved their future concerns and embraced Reagan's proposed $26 billion research effort partly because of "what we know of Soviet capabilities and interest in the field."

"It is well known that certain other questions arise in the future, and nobody suggests they do not," the ministers said. "But this is not the moment to try to settle them because we don't know what the research will bring."

British and West German leaders previously have distinguished between support for U.S. research work and possible deployment, warning that movement toward antimissile defenses could undercut western deterrent strategy.

France, a NATO member but not part of the alliance's integrated military command, has expressed similar worries, and Defense Secretary Caspar W. Weinberger will visit France for three days this week.

Weinberger told reporters that European officials voiced "some reservations" about the research program, chiefly fears that their nations would be left to fend for themselves if the United States successfully deploys an antimissile shield over its own territory.

Weinberger termed the concerns "misunderstandings which I seemed to be able to clear up."

If a space-based defense proves feasible, Weinberger said, experts believe that it would be easier to intercept and destroy Soviet intermediate-range ballistic missiles aimed at Europe than to counter longer range weapons directed at America.

Weinberger had come to Luxembourg hoping for an endorsement of "Star Wars" to bolster both U.S. arms control negotiators in Geneva and administration lobbyists in Congress who are seeking $4.6 billion in research funds for the project in fiscal 1986.

"We were completely satisfied with the results because the things we are doing were supported unanimously," he said.

A senior U.S. defense official traveling with the secretary added that the NATO group's blessing will help dash Soviet hopes of heading off "Star Wars" research by splitting the alliance.

In their communique, the ministers said they support research into the system Reagan calls the Strategic Defense Initiative so long as it is "conducted within the terms" of the 1972 U.S.-Soviet treaty limiting development and deployment of antiballistic missile defenses.

The research, according to the communique, "is in NATO's security interest and should continue."

It noted, "with concern," the Kremlin's "extensive and longstanding efforts" in strategic defense, including an antiballistic missile system deployed around Moscow, and concluded that the American program "is prudent in light of these Soviet activities."

The ministers said they welcome the U.S. offer to include European firms in the SDI research work -- an invitation formally delivered by Weinberger yesterday in a move to solidify allied support.

However, Australia, one of 17 nations invited to take part in the Star Wars program, will not do so, Australia's Defense Ministry said Wednesday, Reuter reported from Sydney.

The NATO ministers' communique said they "strongly support" the U.S. position in arms control talks with the Soviet Union in Geneva and urge Moscow to "participate constructively."

It said that NATO is determined to continue deployment of ground-launched cruise and Pershing II missiles but is willing to "reverse, halt or modify" its plan if the Soviets agree in Geneva to dismantle some of their 414 SS20 intermediate-range nuclear missiles aimed at Europe. Carrington said more than 130 of the 572 NATO missiles are already in place.

In Geneva, the Soviet Union made clear Wednesday that it will discuss with the United States only methods of keeping space free of all weapons -- not the Star Wars project or any other space-based system, The Associated Press reported. The Soviet delegation issued a formal denial of a newspaper article that quoted a U.S. spokesman as saying Tuesday's first subgroup meeting "discussed possible anti-missile systems in space."