The British government, which has strongly backed President Reagan's "Star Wars" plan for research on missile defense systems, raised a series of questions and concerns today about where the project might lead and whether it could be managed "without generating dangerous uncertainty."

The questions came in a major policy address here by Foreign Secretary Geoffrey Howe that was cleared by Prime Minister Margaret Thatcher.

Thatcher repeatedly has stressed her support for the research phase of Reagan's plan. But she also consistently has said that any move beyond that into development and deployment would have to be negotiated with Moscow because of the 1972 U.S-Soviet treaty limiting antiballistic missile (ABM) defenses.

Sources here said that there was no change in policy intended in Howe's speech but that the second part of Thatcher's message tends to get lost in headlines about her support for research and because London felt that the longer term political questions were not getting a sufficient public airing.

Allied diplomats here said his speech was the most important and comprehensive list of questions and concerns raised publicly by any top allied official thus far.

The timing of his address, within days of the opening of new U.S.-Soviet arms reduction talks in Geneva and of visits by Howe and Thatcher to Moscow, underscores the importance the British government attaches to a full discussion within the West now about "questions so vital to our future that we cannot afford to shrug them off," as Howe put it.

He made clear his fear that technology was in danger of overrunning political debate.

"Can we afford even now simply to wait for the scientists and military experts to deliver their results at some later stage? Have we a breathing space of 5, 10, 15 years before we need to address strategic concerns? I don't believe so," Howe said.

The history of weapons developments shows that research and study of its implications must go hand in hand, he said; otherwise "research may aquire an unstoppable momentum even though the case for stopping may strengthen with the passage of years."

Allied and U.S. diplomats said they suspected that Howe's speech and its timing will not be received warmly in Washington, although he sought to present his questions in a balanced way.

While reiterating British backing for U.S. research into possible futuristic antimissile defenses to keep pace with Soviet research, the overriding tone of the 45-minute address to the Royal United Services Institute was one of concern about what might happen after that research phase ends.

"Could the process of moving toward a greater emphasis on active defenses be managed without generating dangerous uncertainty?" Howe asked.

He repeatedly emphasized the importance of the ABM treaty and the agreement made by Thatcher and Reagan last December that any movement of Reagan's Strategic Defense Initiative (SDI) beyond research into development and deployment "would have to be a matter for negotiations."

Both U.S. and allied sources here also say part of the reason for Howe's speech was probably a feeling that there were too many different interpretations of that December agreement in Washington and too many statements indicating that the Star Wars project would go ahead no matter what.

Even as a research program, Howe said, the SDI is "also full of questions. Even if the research shows promise," he added, the key question will remain how best to enhance deterrence of an enemy missile attack and "how best to curb rather than stimulate a new arms race." He said mutual assured deterrence using offensive systems has worked well and should not be discarded lightly.

"It would be wrong to underestimate the enormous technological expertise and potential of the United States," he said. "But there would be no advantage in creating a new Maginot Line of the 21st century, liable to be outflanked by relatively simpler and demonstrably cheaper countermeasures" that the Soviets might develop.

"If the technology does work, what will be its psychological impact on the other side?" Howe said, referring to the Soviets. Reagan, he notes, says he does not seek superiority. "But we would have to ensure that the perceptions of others were not different" to forestall an all-out offensive missile race.

Assuming that limited antimissile defense around key command and offensive missile installations, rather than Reagan's total shield concept, prove possible, Howe asked whether this might not stimulate Moscow to go back to targeting missiles on cities. He quoted Reagan himself as saying in 1983 that a mix of offensive and defensive systems "could be viewed as fostering an aggressive policy."

Would antimissile defenses work, and be survivable against attack, he asked, and "how would protection be extended against the nonballistic missile nuclear threat" posed by cruise missiles and bombers? If only limited defenses were feasible, would these be more vulnerable to being countered and "would these holes in the dike produce and even encourage a nuclear flood?" he asked.

"Might we be better advised," he went on, to put money into hard-to-hit mobile and submarine-based missiles rather than missile defenses? And would such high-tech defenses "permit adequate political control" or would "the world rest solely upon computers and automatic decision-making."

We must also ask, Howe said, not only whether the West can afford the enormous costs involved, but whether there are more effective and affordable ways to enhance deterrence. "How far will we be able to impose new burdens on defense budgets already under strain, and what would be the effect on all the other elements of our defenses on which western security will continue in large part to depend?" he asked.

Howe said Moscow has indulged in a "lot of dogmatic statements and preconditions" for the Geneva talks and undoubtedly will not hesitate to try to split the West and achieve its objectives without concessions.

He said the Soviets will fail if they try, but he also said it was important to convince the new Soviet leadership that the West was serious in maintaining a balance at significantly lower levels of nuclear arms. "We do not want to give them the impression that we have something else in mind."

He said the NATO alliance must be assured that an American missile defense would not erode the U.S. nuclear guarantee to Europe. A missile defense would take many years to deploy and "many years of insecurity and instability cannot be our objective," he said.

He referred several times to Reagan's plans as the president's "vision" and said "we must be especially on our guard against raising hopes that it may be impossible to fulfill. We would all like to think of nuclear deterrence as a distasteful but temporary expedient. Unfortunately, we have to face the harsh realities of a world in which nuclear weapons exist and cannot be disinvented."

Howe put heavy emphasis on the link between offensive and defensive missile forces and said that while efficient defenses would be welcome, "we also have to consider" how a Soviet offensive build-up might proceed "if unconstrained competition in defenses beyond the ABM treaty limits were to be provoked."

If offensive missile forces can be reduced, he said, then the case for defenses against a smaller force may be strengthened. Conversely, he said, radical cuts in offensive missiles might make the need for defense superfluous. Thatcher's earlier agreement with Reagan said the objective should be security at reduced levels of offensive forces, and there is allied concern that U.S. deployment of a Star Wars defense would spur a new offensive missile race.