Britain's nuclear weapons policy has emerged as one of the major disputes of the current national election campaign, with the country's close strategic ties to the United States under the most serious attack in the nuclear age.

Debate over continuing Britain's nuclear role has sharply focused the political differences between Prime Minister Margaret Thatcher's Conservatives and their opponents. The opposition questions whether this once-formidable imperial state, now a medium-sized European power, should maintain its own costly nuclear deterrent as a supplement to NATO's American umbrella.

As in West Germany's elections in March, British voters are expected to make their choices in balloting on June 9 primarily on economic concerns and the personalities of the party leaders. But here as in Germany, the nuclear issue also has developed into an important feature of the campaign because of the impending deployment of new U.S. medium-range missiles.

In Britain's case, the controversy extends to the plan for purchasing $10 billion worth of U.S. Trident D5 submarine-launched missiles over the next decade to modernize the country's aging Polaris deterrent.

It is already clear that a defeat for the government--although considered unlikely--would ensure serious problems for the Reagan administration in its arms negotiations with the Soviets.

In addition, the nuclear debate reflects the bitterly divisive tone of the campaign so far. Conservative spokesmen have accused opposition Labor Party candidates of supporting "Marxist socialism" and issued circulars suggesting that the Campaign for Nuclear Disarmament, the main antinuclear organization, is backed by communists. In her first campaign speech last night, Thatcher invoked last year's Falklands war in declaring that the nation's defense has "the first call on the nation's resources."

"If a hostile government was tempted to pursue its demands by armed aggression," she declared, "which example would be more likely to make it pause: the renunciation of the means of national self-defense . . .? Or, the swift and sure response of our young men in the South Atlantic just a year ago?"

By contrast, Labor under its left-wing leader Michael Foot is firmly committed to abolishing unilaterally Britain's nuclear deterrent, which relies heavily on U.S. weapons technology, and is pledged to oust all American bases from Britain.

"We are going to get rid of the nuclear boomerangs that cannot defend us or anybody," Foot asserted in his opening speech of the campaign yesterday.

Accusing Thatcher's Tories of "smears, sneers and scares" in the nuclear controversy, Foot said: "The trouble with the superpowers is they have not got super brains, and some other countries have to provide for the deficiency." Unilateral British disarmament, he contended, would open the way to broader moves to curb nuclear arms.

Some Labor politicians, particularly the party's deputy leader Denis Healey, are known to believe that so drastic a disarmament would be a serious blow to Britain's security. But the fact remains that the Healey view has been consistently defeated in party councils in recent years, and Labor's official doctrine is now more firmly antinuclear than at any time in its history.

By any reckoning, the consequence of a Labor victory next month would be an upheaval in longstanding American policy for defending Europe and the reshaping of assumptions about British reliability as an ally that date from the early days of the Atlantic partnership in World War II. The sharing of nuclear know-how between the United States and Britain for the maintenance of the latter's separate nuclear force is the only link of its kind in the world and is a cornerstone of the Anglo-American "special relationship."

Even the centrist Alliance of Liberals and the new Social Democratic Party supports a dramatic reversal of Britain's present nuclear policies. It is against the planned upgrading of the country's nuclear arsenal by replacing the existing Polaris missiles with the Trident D5s and takes a skeptical stance about the deployment of U.S. cruise missiles in Britain beginning later this year.

Any changes along these lines, if implemented, would represent a setback to the Reagan administration. They would be likely to increase the incentive for the Soviets to hold out longer in Geneva talks on the medium-range missiles awaiting further shifts in Europe's volatile public sentiment about the upcoming nuclear deployments.

Given the radicalism of Labor's position and the other opposition party views, the British election is potentially even more pivotal to U.S. interests than the West German contest, which pitted Chancellor Helmut Kohl's victorious center-right coalition against the Social Democrats, whose equivocal stance on nuclear deployment was similar to that of their British counterparts.

The reassuring difference to U.S. officials is that at this stage of the German campaign, the parties were more evenly matched in the public opinion polls than is true here. The Conservatives hold a lead over both Labor and the Alliance strong enough to permit forecasts of a possible Thatcher landslide. But pollsters warn against prematurely dismissing Labor's chances or the possibility that none of the parties will gain an overall majority.

In any event, Thatcher has been forced on the defensive on the nuclear issue and warned this week that the Soviets are closely monitoring the British election before making their next moves at Geneva. Lambasting Foot in a radio interview, Thatcher said:

"You do not, if you really hate nuclear weapons as I do, you do not say we will have one-sided diasarmament and throw out all the American bases, leave all the weapons in the hands of our sworn enemies, then hope to goodness they will negotiate. Of course they won't."

Thatcher is also adamantly opposed to the other aspect of the current challenge to Britain's nuclear status, Soviet leader Yuri Andropov's insistence that British and French missiles be included in East-West negotiations over the European nuclear balance. Britain maintains that its missiles--now capable of hitting only 64 targets--are a "last resort" weapon were it to be threatened while its allies were unable or unwilling to come to its aid.

But playing perhaps to public uneasiness over refusal to discuss reductions under any circumstances, the government has modified its stance to allow for some future cutbacks.

"If there is a substantial change in the strategic situation and a willingness on the Russian side to reduce arms," Foreign Secretary Francis Pym said this week, "we have indicated that we would be prepared to reconsider our position."

Short of unilateralism, some Thatcher critics, especially David Owen, deputy leader of the Social Democrats, maintain that Britain should insist on a "dual key" arrangement--requiring both U.S. and British approval before arms use--if U.S. cruise missiles are to be deployed here. The United States and Britain already share control over some of the short-range tactical nuclear missiles and artillery based in West Germany.

But after reportedly discussing the issue this week by telephone with President Reagan, Thatcher reaffirmed that such a step is unnecessary. She expressed the strongly held belief that the terms of nuclear cooperation between the United States and Britain are as satisfactory today as they have been for decades.

"The effect of these understandings," she said in Parliament, "is that no nuclear weapon would be fired or launched from British territory without the agreement of the British prime minister."