A sizable majority of West Germans and Britons believe their national security and way of life depend on U.S. military support, according to a recent Gallup poll, but pluralities in both countries are opposed to increased backing for the United States in disputes with the Soviet Union.

Results of the poll, which was commissioned by The Washington Post and was also conducted in France, come at a time when the allied governments in Europe are deciding whether to back Carter administration sanctions against Iran and strong retaliation, including an Olympic boycott, against the Soviets for their invasion of Afghanistan.

While the poll did not solicit opinion directly on the issue of sanctions against Iran, U.S. analysts observed that the reluctance expressed on entanglement with the Soviets over the Afghanistan issue also would come into play about following the U.S. lead on issues in the Persian Gulf, where European economic interests and oil supplies are at stake.

Both U.S. and European officials acknowledged that the opinions expressed in the poll tend to support a growing feeling among the American public, as evidenced in surveys conducted in this country, that the allies are looking for a "free ride" in which they share the benefits, but not the costs, of a strong U.S. defense posture.

While European diplomats here argued that disagreements on particular policies do not indicate a lack of basic solidarity and concurrence of interests, several U.S. analysts noted that the responses go to the very heart of the strength of the Atlantic alliance itself.

"It just shows [the alliance] may still mean something in the context of European" security, said one senior congressional aide in foreign affairs.

"But the benefits of detente are obviously so strong for the Europeans that we just can't count on their support against the Soviets anywhere else in the world."

A senior State Department official said he could not comment on the survey results "for obvious reasons," including a European Common Market foreign ministers' meeting scheduled early next week to decide the Iranian sanctions questions.

But he pointed out that the United States, France and West Germany are all in the middle of election years, and that governmental decisions are more attuned than usual to strong public opinion. "All I'll say is buckle your seat belts. It's going to be a rough voyage this year," he added.

In March, Gallup asked 1,068 Britons, 1.009 Germans and 1,000 Frenchmen: "Would you say that U.S. military support is essential to our nation's security and way of life, or would you say that we are in a position to do without U.S. miltary support?"

Seventy percent of the British and 61 percent of the German respondents said such support was essential. In France, which is part of the NATO community but does not belong to its military partnership, only 24 percent agreed.

A second question asked: "Which of these two statements do you tend to agree with more: 'Our government should back the U.S. against the Soviet Union more than it has until now, or . . . should do everything possible to stay out of arguments between the U.S. and the Soviet Union'."

Fifty-five percent of the Britions, 66 percent of the French and 37 percent of the Germans said their government should do everything possible to stay out of U.S.-Soviet arguments.

Another question in the poll asked: "Do you agree or disagree with this statemennt: 'Great Britain/France/West Germany should boycott the Moscow Olympics because of the Russian intervention in Afghanistan'?"

Only in West Germany -- whose government favors the boycott -- did a plurality, 41 percent to 31 percent, agree.

Sixty-two percent of the Britons asked disagreed with their government's announced decision in favor of the boycott, agreeing with the British Olympic Committee's opposition to it.

In France, which has held no public decision on the boycott, 56 percent were oppossed to it.

Asked for opinions and interpretations of the responses from their countries, European dipolmats here made several points. All stressed their belief that the Atlantic alliance remained firm, and questioned a U.S. public tendency to equate policy disagreements with fundamental differences in longterm goals and strategies.

"I'm not surprised that so many said U.S. military support was essential," said one German diplomat. "I would have thought it would have been even higher than 61 percent. That's something that hasn't changed since the end of World War II."

But there is "a difference between feelings toward the United States, and toward the present administration," he said. In Germany, "there is a great deal of irritation, because people don't understand these rather abrupt policy switches" by the Carter administration. This diplomat, and others, pointed in particular to the administration's off-again, on-again campaign for economic sanctions against Iran.

A French diplomat aruged over the use of the word "more" in the question asking if French citizens thought their government should "back the U.S. against the Soviet Union more than it has until now."

"You'll say I'm a sophist," he said, "but it's pretty important. If you asked the French whether they support the United States or the Soviets, you'd get a different answer.

"We don't think that [U.S.] policies are always the most appropriate" for dealing with crises in Iran and Afghanistan, he said. The French diplomat pointed out a "great consistency" in his country's foreign policy -- its "independence" of thought; "independence in terms of defense . . . with its own nuclear forces."

Many of the diplomats expressed concern over the perception that the U.S. public does not view them as good friends or allies. But while these same diplomats three months ago, early in the Afghan and Iranian crisis when those perceptions first became apparent, hastened to detail supportive actions their governments had taken, they now appeared irritated at the U.S. reaction.

The British government, one of its diplomats here said, "of course regards the U.S. military commitment to the defense of Western Europe as the pinnacle of our own defense. But it is not a one-sided commitment, I hasten to add. We ourselves, Britain, commit an equally high proportion of our gross domestic product to the military defense of the North Atlantic area."

As for the question of supporting U.S. policies toward the Soviets on the Afghan issue, he said, "We in Britain, as elsewhere in the democratic Western world, feel equally strongly about the Soviet threat. We broadly share the U.S. assesment.

"But of course it does not follow that all countries in the Western alliance will necessarily follow every other country's policies identically. We are an alliance of free partners, not of puppets. And that is what distinguishes us from the Warsaw Pact."

While the French diplomat pointed out that, in a recent poll in his country, the United States was rated France's third best friend, following West Germany and Belgium by only a small margin, he noted that "not wanting to support a policy more does not mean being a good or a bad friend. Sometimes the best friend is one who doesn't agree with you all the time."

The French, he said, "are always sorry, and a little angry, when they have the feeling the Americans don't think we are their friends." In fact, he said, the United States "is one of the few countries in the world we've never had a major war with."