President Reagan's national security adviser said here today that if future research into the so-called Stars Wars antimissile systems indicated that such a defense would work, "we would think it a reasonable subject for discussion, negotiation with our allies and the Soviet Union" before any unilateral American decision to deploy such a system.

At a press conference here, top presidential aide Robert McFarlane said, however, that it would take five to 10 years of research on futuristic space-based defense technology before a judgment could be made about whether a workable defense was possible.

And, he said, "the feasibility of limiting that research" through negotiations "is very much in doubt" because what goes on in the laboratories of both superpowers cannot be verified.

McFarlane was one of several top officials who left the just concluded Geneva talks between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei Gromyko and scattered to key NATO capitals to brief allied leaders on the results of those discussions.

McFarlane had a private one-hour meeting with British Prime Minister Margaret Thatcher. Afterward Thatcher's office said the prime minister appreciated Reagan's dispatch of the top White House aide on such matters and "warmly welcomed" the agreement at Geneva to hold negotiations on nuclear and space weapons.

Thatcher's sentiments were echoed in Brussels by NATO officials briefed by Richard Burt, assistant secretary of state, and other U.S. officials, special correspondent Stephen J. Dryden reported.

The NATO officials "took special note that one of the new negotiating groups would have the objective of radical reductions of intermediate-range nuclear forces," including those that are now being deployed by NATO in Western Europe, Burt said at a press conference.

The division of the arms control negotiations into three groups ensures that the question of medium-range missiles, a controversial issue in Western Europe, will "continue to get direct, high-level attention" and not "fall by the wayside," Burt said.

Alliance ambassadors said they were particularly pleased with the speed with which U.S. officials briefed NATO on the Geneva talks, Dryden reported. Burt was accompanied by special arms control envoy Paul Nitze and the negotiator on strategic arms, Edward Rowny.

Burt warned, however, against "undue expectations" for the new talks, and said the same "solidarity, patience and firmness" that the United States believes brought the Soviets back to the negotiating table would be necessary to conclude an arms control agreement.

While in Brussels, Nitze also briefed the Belgian government, which is facing growing political pressure to put off its scheduled deployment of NATO cruise missiles in March.

In Bonn, West German Chancellor Helmut Kohl said he was "cautiously optimistic" about prospects for success in the arms talks, Washington Post correspondent William Drozdiak reported. Kohl warned, however, that the negotiating process would require "much time and much patience" because of the "difficult and fateful" issues involved in the talks. Nitze is scheduled to brief West German officials Thursday on the Shultz-Gromyko talks.

In Washington, the State Department said Nitze is also scheduled to travel to the Netherlands and Burt to Tokyo to continue the briefings of U.S. allies. McFarlane is to go to Italy and France. The spokesman said Jack Matlock of the National Security Council will brief Canadian officials and Mark Palmer, deputy assistant secretary of state, will go to Israel and Egypt.

The spokesman also said Kenneth Adelman, director of the Arms Control and Disarmament Agency, will discuss the Geneva talks with East European officials during a previously scheduled trip to Hungary, Yugoslavia and Romania.

In a London press conference, McFarlane's comments underscored a four-point agreement Thatcher reached with President Reagan during a visit to Camp David last month.

While Thatcher, after that meeting, lent strong public support to the need for U.S. research to balance Soviet research, one of those points stated that any actual deployment of the Strategic Defense Initiative, the official name for Star Wars systems, "would have to be a matter for negotiations" in light of the existing 1972 U.S.-Soviet antiballistic missile treaty.

The day after Thatcher left Camp David, however, Defense Secretary Caspar W. Weinberger said in a television interview that the defense plan offered "the only real hope" for long-term deterrence of nuclear war and added that "we will not give that up." Those comments were viewed here as part of a continuing uncertainty and divisions within the administration over the U.S. arms control position.

A little over a week later, a senior administration official told reporters that Star Wars "is not a bargaining chip." He added, however, that the administration wanted a "thorough-going exchange with the Soviet Union on the full family of defensive systems as well as offensive systems."

McFarlane's comments today also lent strong support to the possibility that advanced technology might produce a nonnuclear defense against missiles that was a better and more benign way to avoid nuclear war than relying only on a balance of terror among opposing offensive missiles.

But he also repeatedly showed sensitivity to European fears that the Reagan administration was bent, no matter what, on unilateral deployment of Star Wars if it looked feasible. There were also hints here from a well-placed British source that Reagan may be more determined to achieve big arms reductions than about ultimate Star Wars deployment.

Under questioning, McFarlane said it was premature to make any judgments about which, if any, technologies, will prove to hold promise as a screen for the United States and its allies against missile attack. That was why years of research were needed.

But he said it is "imperative that sensible, moral leaders try hard, because of the nonthreatening aspect, to make it work."

"Now," he added, "no one has any illusions, surely in the United States or in her majesty's government that these are systems which one would unilaterally deploy without regard for the consequences in the perception of the other side. And we expect to discuss these systems with the Soviet Union so that we develop hopefully with them a common view of how they can contribute to deterrence."

At another point, McFarlane said, "We don't have any illusions that one side or the other could unilaterally deploy these systems and we have no such intention."

Under further questioning, he said the administration did not "have some notion that these systems can be arbitrarily researched, developed and deployed by one side without consideration to how they are viewed by the other . . . and indeed there are treaty commitments and obligations which govern" this.

This was a reference to the 1972 ABM treaty and McFarlane said, "We are firmly committed to observing existing treaty commitments."

That treaty also bans development work and testing. British sources said the question was not raised in the Thatcher-McFarlane talks about how research alone, without development and testing, could provide enough information to judge the feasibility of deployment.

McFarlane, at the press conference, said the U.S. delegation at Geneva had told the Soviets of concerns that Moscow may not be preserving the ABM treaty because of construction of a large radar in Siberia that U.S. officials believe may be a violation.

The Soviets have made cancellation of the U.S. Star Wars and antisatellite programs their number one priority in any new talks. But McFarlane said it was the U.S. objective to engage the Soviets in a dialogue that would try to convince them that defense may be better than offense and to try to find ways that the pace of developments would not raise fears that either side was seeking to gain the kind of advantage that might lead to a clear first-strike capability without fear of retaliation.