The opposition Labor Party today overwhelmingly approved a policy of unilateral nuclear disarmament for Britain and the closing of all U.S. nuclear bases here, despite warnings from three senior party members that such a move could split the NATO alliance abroad and prove politically suicidal at home.

The resolution, passed in a debate punctuated by sharp attacks on American policy at the party's annual convention, called for removal from Britain of all U.S. nuclear weapons -- including cruise and Poseidon submarine missiles and F111 bombers -- and the closing of all bases for such weapons.

Although several speakers made demands to "close all American bases in this country" and "get Americans out," a second resolution calling for the closing of all U.S. bases here, including those not connected with nuclear weapons, was defeated by about a 2-to-1 margin by the delegates, who represent more than 6 million trade union members and 300,000 party members.

There are no immediate prospects for the Labor Party to regain power, which it lost in 1979 to the Conservatives, led by Prime Minister Margaret Thatcher, who strongly supports retention of Britain's independent nuclear deterrent and the new U.S. missiles.

The next election is at least three years away. The Labor Party, however, has pulled itself up in opinion polls in the past year to close to the ruling Conservatives, although two recent public opinion polls show Labor's standing has declined again to 6 percentage points below the Conservatives. But the polls also show that only 41 percent of British voters are satisfied with Thatcher's performance.

It is widely held that Labor's stance on defense was a leading cause of the party's drubbing in the 1983 election. Despite this, however, it is clear that the majority of the party is now even more adamant on leading the fight for nuclear disarmament and that they feel the new policy is more clear and logical than last year's.

Former Labor prime minister James Callaghan, who was booed at this conference last year when he warned against a similarly stark defense policy before Labor suffered its worst defeat at the polls, told the delegates, "None of us knows what the political consequences will be."

However laudable the idea of nuclear disarmament, he said, he could not support taking such a "unilateral step" that could have a "profound effect upon our allies" and destabilize the present fragile balance of power.

"We are not Holland, Belgium or Denmark," Callaghan said last night at another meeting. "We are one of the most significant members of the western alliance." The international repercussions could "make people's flesh creep," he said.

Denis Healey, former defense minister and now foreign affairs spokesman for the party, said that while he supported getting rid of new U.S. missiles, he did not advocate Britain ridding itself of its own Polaris submarines -- as the new party platform demands -- without negotiations. He urged the party to think long and hard about taking steps that would lead to a U.S. troop pullout.

Introducing Callaghan, who is extremely unpopular with the party's left, convention chairman Eric Heffer told the audience not to hiss and boo. But there were some hisses for both speakers. The dominant tone of the debate was highly critical of American policy and those statements met with strong applause from most of the 1,300 delegates.

Gavin Laird, general secretary of the electrical workers union, was the only speaker to challenge official policy directly and he was hooted when he said it was Soviet troops that were in Poland, Afghanistan and Czechoslovakia.

Laird called for multinational nuclear disarmament to involve the Soviets and he called the party's position "a sham that the electorate will see through."

Peter Shore, a shadow cabinet minister, said in another meeting last night that Labor's defense policy last year had been clearly understood and clearly rejected by British voters and that there was no issue on which the ruling Conservative Party lead over Labor was greater than on defense.

The new party chairman, Neil Kinnock, is a leading architect of Labor's unilateral approach.

The opposing statements today, however, make it clear that Kinnock also faces doubts in the center of his left-of-center party, as well as some challenges from the far left that surfaced earlier in this convention.

Although the Labor Party pledged to retain a strong conventional defense, Shore, in making his case last night, said that to rely only on conventional defense was to ignore the example of Hiroshima and Nagasaki. Japan, he noted, was "armed to the teeth" and retained its martial spirit, but surrendered quickly when the bombs went off.

Today's actions reflect broadening resistance by the rank and file of British opposition parties to the presence of U.S. cruise missiles. Two weeks ago, the Liberal Party, against the wishes of its chairman, David Steel, voted to ban the cruise missiles.