President Reagan's determination to press ahead with research on a futuristic, space-based defense against missile attack is causing anxiety and confusion among U.S. allies in Europe, even though it is acknowledged to be playing a valuable role in getting Moscow back to at least preliminary arms control talks.

The anxiety stems mostly from the fact that while Europeans do not want the so-called "Star Wars" project to go beyond the research stage and want it used as a bargaining chip for new agreements with Moscow, Reagan and Defense Secretary Caspar W. Weinberger really appear to believe in the project and want to see it ultimately developed, tested and, if it seems to work, deployed.

The confusion among allied officials is over whether the project, officially called the Strategic Defense Initiative, or SDI, will be placed on the negotiating table, or is inviolate. European officials say it is still "very difficult for European governments to fathom where the U.S. is going on this issue," as one West German put it.

The allies generally accept the idea that the United States must keep up research in vital areas such as antimissile defense, especially when the Soviets have active programs.

British, West German and French officials also made it clear in interviews that the allies understand that Soviet concern about U.S. antimissile and antisatellite projects probably was the crucial factor that caused Moscow to resume preliminary arms dialogue with Washington at Geneva next week.

Thus, specialists say, the SDI has emerged as what one German official called a "trump card" that gives Washington some leverage at any forthcoming arms negotiations.

It becomes an even better trump card with European backing, he added.

But allied officials stress that the backing at this point is for research, not development and deployment, and that the trump card should be played to reach an accommodation.

The dilemma for the allies, as one British official put it, is how to use the leverage that Star Wars may offer now without losing the ability to stop the project later.

A number of allied officials voiced private concerns that once the $26 billion for the research program begins to flow heavily, the momentum and financial pressures will become hard to control and, as one official put it, "the research label will inexorably be exploited, work will go well beyond it, and all of a sudden, there we are."

Furthermore, officials say they see no one within the administration who might seek to put a brake on development or deployment of Star Wars.

On the other hand, the project is so personally identified with President Reagan, and involves a defense that may be 15 years away from reality -- if it ever materializes -- that it could be difficult for a successor administration, even a Republican one, to sustain support for the controversial project.

"I told the president of my firm conviction that the SDI research program should go ahead," British Prime Minister Margaret Thatcher said after a Camp David meeting with President Reagan on Dec. 22.

However, virtually all of the public commentary by allied leaders, including Thatcher, on space weaponry and defense have been negative.

These comments have been in sharp contrast to the vision that Reagan and Weinberger have that antimissile defense may work and is a better alternative than relying on a balance of terror from unchecked offensive weapons.

Thatcher has expressed a view shared by the West Germans and the French, that a space arms race must be avoided and that when one side goes beyond research, "the other will surely follow and within but a short time we shall have the same military balance but at a higher level and at a higher cost."

Last month, French President Francois Mitterrand suggested that the SDI amounted to "overarmament," and last July France's then-foreign minister Claude Cheysson criticized it as "a Maginot line" in space.

In her meeting with Reagan, Thatcher "hammered out," in her words, four points of agreement with Reagan on space that European officials believe could become an important benchmark of presidential commitment, despite apparent differences within the U.S. administration about the long-term future of Star Wars.

Those four points state that any actual SDI deployment would "have to be a matter for negotiations," and that the overall aim "is to enhance, and not to undermine, deterrence."

The British leader has made known her belief that the existing policy of mutual deterrence has worked well.

But the allies, officials said, are still worried that the system will not work, and will drive the Soviets into a new round of offensive and defensive weapons development that could leave Europe "out in the cold," as one official put it.

This was a reference to the prospect that a missile screen over the United States would be viewed in Europe as an American attempt to decouple itself from Europe, which would not be protected, or at least not as well as the United States.

There are also concerns that a superpower race for missile defenses could nullify the existing British and French independent deterrent force of offensive nuclear missiles.

There also are concerns that the appearance of American intransigence on space weaponry at any resumed negotiations could provide ammunition to the Soviets that could reawaken the antinuclear and peace movements in Western Europe and bring renewed pressure on Belgium and the Netherlands not to accept their share of U.S. cruise missiles now being installed in Britain, Italy and West Germany.

Indeed, one of the most extraordinary things about the current and relatively sudden international focus on Star Wars and U.S. and Soviet military space programs is that it has totally overshadowed the initial deployment of those cruise and Pershing II missiles little more than a year ago, an event that brought considerable political turmoil to Europe.

Allied sources say the focus on space is a double-edged sword. It tends to hold down any lingering political controversy over the missile deployments, but it also threatens to lead governments to forget that the Europeans still want to negotiate reductions in the threat posed by growing numbers of Soviet SS20 medium-range missiles that the new U.S. missiles were meant to counter.