French and West German leaders sought today to coordinate a European response to President Reagan's "Star Wars" missile defense proposals amid fears here that the U.S. initiative could undermine public confidence in nuclear deterrence.

Today's talks between French President Francois Mitterrand and West German Chancellor Helmut Kohl formed part of a series of regular summit meetings devoted to strengthening the Bonn-Paris relationship within the European Community. They also underlined subtle differences of approach between the principal U.S. allies toward Reagan's Strategic Defense Initiative (SDI), as the "Star Wars" program is formally known.

While Western European leaders share similar concerns about the long-term impact of Reagan's plan on the East-West nuclear balance, they differ in their assessment of how to respond to the technological and scientific challenge posed by huge U.S. investments in space research.

Traditionally preoccupied with developing an independent defense strategy, France appears more committed than either Britain or West Germany to promoting joint European projects in space rather than joining in U.S. research efforts.

Before coming here, Kohl suggested in an interview with Die Zeit, a leading West German weekly, that it would be difficult for West Germany "to stay on the sidelines of such a technologically and economically important process." He added, however, that West German participation in SDI-type research could "be considered only if there is full access to the economic benefits."

A similar willingness to join U.S. research efforts was expressed by British Prime Minister Margaret Thatcher during a visit to Washington last week. She told Congress, "I firmly support President Reagan's decision to pursue research into defense against ballistic nuclear missiles -- the Strategic Defense Initiative." She said she hoped British scientists would share in the research.

French officials have shown more skepticism about the benefits of SDI while conceding that the technological gap between Western Europe and the United States could widen if the Reagan administration succeeds in devoting vast research funds to the development of an antimissile defense system in space. The French solution is to encourage other European countries, particularly West Germany, to compete with the United States by adopting joint research projects of their own.

At a press conference tonight, Mitterrand reiterated hints made earlier in the week that he will take major "new initiatives" toward greater European unity during the next few months in consultation with Kohl. He listed development of a joint defense strategy as one of a number of possible areas for European cooperation.

"Other countries are advancing at a great pace. If Western Europe wants to participate in this race , we must prepare ourselves. There is not a single day to lose," he said, striking a theme that is considered likely to form a central part of his Socialist government's attempts to reverse unfavorable opinion polls before legislative elections next year.

Kohl noted that the United States had not extended any "official invitation" to European governments yet to participate in SDI research.

"If there is a formal invitation, we will study it together with France and with other European governments as well," he said, adding that the "technological, economic and industrial" implications of SDI needed to be examined extremely closely.

The French-German consultations coincided with a trip by Soviet Foreign Minister Andrei Gromyko to Italy and Spain in which he has been urging his hosts to act to prevent an arms race in space. Italy has so far taken a less clear stand on SDI, with Prime Minister Bettino Craxi saying that it should be studied "without prejudice or excessive distrust," and, in an interview published today in The New York Times, that it should be a matter of intensive talks at an early stage of the U.S.-Soviet arms negotiations in Geneva next month.

In private, some Western European officials have expressed fears that the Kremlin could be laying a diplomatic trap for the West by holding hostage any agreement on intermediate and long-range nuclear weapons in Geneva to U.S. concessions in space.

In recent weeks, French officials have said that their main objection to SDI is that it could undermine public support for the existing 30-year-old strategy of nuclear deterrence based on the concept known as "MAD" -- mutually assured destruction. They fear that Reagan's talk about a world in which nuclear weapons are rendered useless could reinforce the arguments of pacifist groups in Western Europe.

Noting the failure over history of attempts to construct a foolproof defensive system, one French official said that experts agreed that no antimissile shield of the type envisaged by Reagan could ever be "100 percent effective." French officials insist that there is no question of France's nuclear strike force -- the force de frappe -- becoming obsolete.

"We are not worried that this will render our own independent nuclear deterrent useless. What concerns us is the impact on public opinion, particularly in countries like West Germany, where there is not such a strong national consensus on defense issues as there is here," a French official said.

While welcoming signs of a shift in U.S. emphasis away from the development of a totally effective "shield" to a more limited protection for missile silos, Mitterrand's foreign policy experts are confused by what they see as conflicting signals from the Reagan administration. "The same U.S. official can tell us one thing one day and something quite different three weeks later," one official complained.

There has been a similar confusion in some recent French statements, with Defense Minister Charles Hernu warning of U.S.-Soviet "complicity" in the strategic arena and Foreign Minister Roland Dumas insisting that SDI poses no particular anxiety to France or Britain.