An effective space-based defense against missiles cannot be built for at least a quarter-century, and even then could be pierced by the relatively small French nuclear arsenal, a French government panel has concluded.

The panel of scientists and defense experts said that President Reagan's Strategic Defense Initiative, sometimes called "Star Wars," and a similar Soviet research program will require many "spectacular technological breakthroughs." Any deployment within the next 25 years "would be very improbable," the commission said.

Even if a space-based shield is deployed, France likely could penetrate a superpower defense, either by modifying its arsenal or by attacking defensive weapons in space, the commission said.

"It should be noted that a space defense system itself will no doubt be fairly vulnerable," the French report states. "A medium-size power like France could probably secure the means with which to partially neutralize the space defense system, or impair its performance sufficiently to assure penetration by its ballistic missiles."

The French experts nonetheless said the research program is likely to benefit the United States. A partially successful defense by one superpower could complicate the other's plans for nuclear attack, and the research will yield substantial technological advances in other areas, they said.

"This undertaking has certainly reinforced U.S. prestige, because it confirms its status as a dominant technological power," the report says.

"The deployment of a space defense system is not without interest to the two great powers because it renders the outcome of a first . . . strike less certain," the report adds, "and in general provides them with the means for controlling space."

Still, the panel's relatively modest expectations for SDI conflict with official U.S. positions in key respects. The Pentagon's SDI office declined to comment on the report, but Lt. Gen. James A. Abrahamson, SDI chief, told a gathering at the National Press Club yesterday that he is confident an effective defense can be developed.

Abrahamson said his program is accomplishing a decade's worth of research in five years, with "breakthroughs" already in laser technology and in developing sensors that can detect enemy missiles even in an atmosphere clouded by radiation from exploding nuclear bombs.

The French panel was convened to analyze whether the superpowers can build defenses that would render the much smaller French nuclear force ineffective. Its report, completed Jan. 30, was coordinated by J.F. Delpech, research director of the National Center for Scientific Research, with contributors from the Defense Ministry, atomic energy commission and other French government agencies.

French defense officials may draw on the panel's advice as they plan the modernization of their nuclear force, a French official said. A conservative French government, elected since the report was submitted, is expected to continue the previous government's policy of maintaining an independent nuclear force while keeping a distance from SDI, but what impact this report may have on policy is unknown.

Sen. Dale Bumpers (D-Ark.), an SDI skeptic, asked the French Embassy for a copy of the report after reading a small item about it in Defense News, an aide said. Bumpers then asked the Congressional Research Service to translate the analysis, and that 15-page translation was used in preparing this article.

The French nuclear arsenal, now being expanded, contains about 500 warheads, generally targeted at Soviet cities. The United States and Soviet Union together have about 50,000 warheads.

"Even in the case of extremely rapid technical progress, accompanied by considerable appropriations," the report says, "it does not seem very realistic to expect that one of the great powers will deploy a space component, sufficiently large to be reasonably effective, before the year 2010."

In the meantime, the panel said, the French Defense Ministry should develop a force that could penetrate Soviet defenses, such as by building smaller missiles in larger numbers, releasing them in concentrated volleys, hardening them against laser attack and shooting them in short, flat trajectories. France also should develop jammers, laser reflectors and nuclear warheads that could temporarily blind sensors in space, the panel said.