Britain's opposition Labor Party voted overwhelmingly today to abolish the country's nuclear weapons arsenal if the party wins the next election and to reject the deployment of any American nuclear missiles in Britain.

The decision at the party's annual conference represents a potentially major blow to Western nuclear defenses. Unlike previous such resolutions, this one obligates the party leadership to make antinuclear policy a feature of its campaign platform. Announcement that the resolution had won more than a two-thirds majority was greeted with a standing ovation and prolonged cheers from the delegates.

The Labor Party also voted decisively, as it has done before, not to withdraw from the North Atlantic Treaty Organization.

Reacting for the Conservative government, Deputy Foreign Minister Douglas Hurd said Labor's votes undermined the NATO alliance. "You cannot stay in NATO and reject NATO's strategy for peace," which is based in part on a British nuclear deterrent, he said.

The Labor Party's antinuclear policy underscored the chasm that exists between the Reagan administration and the main opposition party in Britain. Differences with the United States extend across the board. Speaking yesterday, party leader Michael Foot repeatedly linked his denunciations of Prime Minister Margaret Thatcher with an attack on Reagan administration policies, specifically on economic issues.

"Reaganism is like Thatcherism and the other way around," he said. "The combination of the two together is what threatens the world on a scale that we have not known for generations." Through arms sales "fed by the West," he said, Britain and the United States are responsible for "some of the worst evils of the Third World."

There is little likelihood of Labor being voted into power in the immediate future. Thatcher's popularity has increased since Britain's victory in the Falklands war, according to polls. Elections, required in 1984, are not expected before late 1983.

Western Europe's other nuclear power, France, maintains a nuclear force separate from NATO. While the government of President Francois Mitterrand is socialist, like the Labor Party, it takes a much stiffer line than Labor on the perceived dangers to security posed by the Soviet Union and the need for a strong defense.

In today's disarmament debate, only one speaker, Sir John Boyd of the Electrical Workers' Union, specifically mentioned the threat posed to Britain by the Soviet Union's nuclear strength. He was roundly heckled. Instead, speakers focused on the dangers of nuclear warfare, the need to renounce a British arsenal and resistance to the deployment of any nuclear weapons in Britain by the United States.

Joan Lestor, an outgoing member of Labor's National Executive Committee, dismissed the Thatcher government's support for the position advocated by the United States of deep cuts in both medium-range and strategic nuclear weapons. "We are interested in a zero option whereby no nuclear bases in Europe or outside exist," she said, "a true zero option, not Reagan's one-sided cosmetic approach."

What the policy adopted today would mean, should it be fully implemented, is cancellation of Britain's plans to acquire the U.S.-made Trident D5 submarine-launched missile system to replace the existing Polaris force and repudiation of Britain's agreement to station cruise missiles as well as other weapons with American crews here.

The resolution also calls for substantial reductions in Britain's defense spending, now the highest of any European member of NATO measured in percentage of gross national product. The vote total, reflecting a complex system of balloting to represent local party organizations and trade unions, was 4,927,000 in favor of unilateral disarmament and 1,975,000 opposed, a substantial increase over the tally of similar proposals in past years.

The conference also adopted a resolution calling for full nationalization of the country's arms industry, but the margin was narrow and the National Executive Committee favors a less sweeping proposal as a campaign platform.

The party is also committed to withdrawal from the European Community, a policy generally reaffirmed by Foot yesterday. Nonetheless, by stressing that withdrawal would be "in conformity" with Britain's international obligations, Foot seemed to be acknowledging Labor's interest in maintaining the best possible relations with Europe, especially with socialist governments in France, Greece and--if it survives its present political stresses--West Germany.

The vote on nuclear weapons was the most unequivocal foreign policy position of the conference and a move widely portrayed as "historic" in the long, emotional struggle over disarmament policy in the Labor Party. The clear mandate of the delegates is for a Labor government to take a stance like Canada's or Norway's in renouncing any nuclear force while remaining inside NATO.

One of the party's leading antinuclear activists, Lord Brockway, noting that he would be 94 next month, said that, "except for the campaign" to dismantle Britain's colonial empire, "I have never known a movement so strong throughout the world as the movement for disarmament today."