U.S. allies in Western Europe strongly oppose the Reagan administration's intention to end American observance of SALT II treaty limits on strategic nuclear weapons and have expressed hopes that the decision made public in Washington Tuesday will not be implemented.

Britain, which has taken the leading role in allied opposition, said in a statement that it "would much regret it" if the United States went ahead with its "provisional decision" to exceed arms limits set out in the unratified SALT II accord.

A British official said that the U.S. announcement had come as no surprise and that high-level discussions with the administration on the overall issue of treaty compliance have been going on for the past month, including at the Tokyo summit meeting of industrialized democracies last month.

West german Chancellor Helmut Kohl is known to have written two personal letters to President Reagan -- one before the Tokyo session and one afterwards -- urging him to stay within the bounds of the treaty. British Prime Minister Margaret Thatcher's "views on the subject were made very clear," the British official said.

"No one in Washington could have been under any illusion as to the position of top allied leaders," he said.

The White House announced Tuesday that the United States would take two Poseidon submarines out of service this spring to avoid exceeding the treaty limits with the deployment of a new Trident-class submarine. But it added that unless the Soviet Union moved to rectify its own alleged treaty violations, the United States would no longer be bound by SALT II. Planned U.S. deployment of its 131st cruise missile-armed B52 bomber this fall would exceed treaty limits.

One official here said that the announcement was "arguably ambiguous, but less so than it appears." Britain and the other European allies were under no illusions about the firmness of U.S. intentions, he said. "If you read the fine print, . . . you have to interpret it as an intention not to abide" with the treaty terms.

That interpretation, was bolstered yesterday by Defense Secretary Caspar W. Weinberger, who said Tuesday's announcement meant that the United States is "no longer bound" to continue observing SALT II.

But until Tuesday's decision was announced, the Europeans felt constrained from focusing public attention on their position. "The preliminary focus was on the two Poseidons," the official here said, and Britain's official statement "welcomed" the decision to take them out of services.

Now, however, the Europeans have decided to "break out" on the issue of overall SALT compliance and will make it "the main focus" of the NATO foreign ministers meeting that began today in Halifax, Nova Scotia.

Without exception, according to officials here, NATO's European members believe a U.S. decision to end observance of SALT II terms is inconsistent with continuing U.S.-Soviet arms control negotiations in Geneva. While sharing Washington's concern about evidence of Soviet noncompliance with the treaty limits, the allies have been reluctant formally to do away with an existing, if unratified, accord until something is negotiated to take its place.

The allies also fear that the decision could affect prospects for a meeting this year between Reagan and Soviet leader Mikhail Gorbachev. "The Soviets may just decide that the administration is playing hardball. Gorbachev's internal position may be that he can't give in," a British diplomat said.

Neither the question of continued U.S. compliance with SALT II nor allied concern about it is new. Reagan has long labeled the agreement, negotiated by Jimmy Carter, as "fatally flawed," although Washington and Moscow each said they would abide by its terms as long as the other did.

Although the Europeans differ in varying degrees with the U.S. assessment of alleged Soviet violations, Thatcher has raised the issue with Moscow several times, most recently in a letter to Gorbachev carried to Moscow last week by Deputy Prime Minister William Whitelaw.

Britain has said that the alleged violations are "a subject for legitimate concern" by the United States, an official here said. "We've told them they have to respond to that concern."

But in a television interview last week, Thatcher repeated, "I am very anxious that there should be another summit between President Reagan and Mr. Gorbachev, and I recognize that that one has to be a success for both of them, and therefore it must produce some specific advance in arms control agreements. That means we have all got to work jolly hard at Geneva to get that advance."

Officials here said they did not see the U.S. move as a "tactical" one, designed to put pressure on the Soviets, but rather as "logical and consistent" with the thinking within the Reagan administration. "Put in Orwellian terms," said one, "the administration believes that SALT is bad, military buildup is good."

Based on their interpretation of the White House position, officials here made several assessments on the future of both the Geneva arms control talks and the SALT treaty itself. Assuming it is still possible to arrange a summit, they said, it could be a Vladivostok-type meeting, the preliminary 1974 session between Gerald Ford and Leonid Brezhnev that laid out a framework for SALT II.

"In that case," a British diplomat said, "Reagan wouldn't have to eat crow" in backing down from this week's announcement, "and could say that this was a new step beyond SALT II -- or they could have the summit anyway, without prior preparation, in which case SALT II becomes very important" as the existing agreement.

"It all depends on what the Soviets do" in response to the U.S. announcement, he said. "They could play it for all it's worth. They could insist they are in compliance with the treaty, or say that if the Americans are going to exceed the limits, so are they. They could urge substantive Geneva talks. Or they could play hardball and say no summit at all."