n Argentine resolution calling for negotiations on sovereignty over the Falkland Islands won overwhelming support in the General Assembly tonight -- including the American vote -- despite the adamant opposition of Britain.

The final tally was 90 to 12 with 52 abstentions.

The outcome was a major triumph for Argentina, which had sought a public manifestation of international support at the United Nations to salve its injured national pride after the British recapture of the islands last June.

It also was a rare diplomatic defeat for the British, who have not been so politically isolated at the United Nations since the 1956 Suez debacle.

The American vote was a crushing disappointment for the government of Prime Minister Margaret Thatcher, which has sought to strengthen its alliance with the Reagan administration.

But intensive British diplomacy did succeed in averting a massive defection of its European Common Market partners, several of which had initially indicated their intention to vote with Argentina.

In the end, all the Market nations except Greece abstained, as did most other Western nations. The 11 nations joining Britain in voting against the resolution were: Antigua, Belize, Dominica, Fiji, Gambia, Malawi, New Zealand, Oman, Papua, Solomon Islands and Sri Lanka.

The Soviet Bloc and most Arab and African nations joined the Latin American group -- as did Israel -- in voting in favor.

The United States was the only one of Britain's major allies in the Argentine camp, a demonstration of an intrinsic difference in the national interests of the two countries.

American officials here argued that an abstention would have hurt American attempts to mend fences in Latin America more than the "yes" vote damaged the British position. This, they said, was the major consideration in the decision taken Monday by President Reagan, who will embark on a visit to three Latin American countries later this month.

The practical impact of the resolution -- which, like all assembly measures, is not binding on governments -- is minimal.

British Foreign Minister Francis Pym announced in Parliament several hours before the vote that "we cannot accept a call for negotiations on sovereignty after an unprovoked attempt to force the issue by invasion."

He called the resolution "an Argentine-inspired U.N. charade," which "continues to prejudge the substance of the issue."

In an implicit slap at the American stand, Pym added that "the vote at the U.N. may go against us but such a result will bring little credit to those whose memories proved so short."

The resolution, in addition to asking a resumption of negotiations by Britain and Argentina "to find as soon as possible a peaceful solution to the sovereignty dispute," asks Secretary General Javier Perez de Cuellar to renew his "mission of good offices" as a mediator.

And to be certain that the dispute remains alive at the United Nations, the resolution assures that the item will be on the agenda of next year's assembly session.

British Ambassador Sir John Thomson maintained that the real danger of tonight's vote was that it would "encourage the government in Buenos Aires to think that the assembly was prepared to turn a blind eye to their act of aggression, and therefore to think that they could get away with renewed pressure on the Falklands."

He charged that "the Argentines continue to make it clear that they expect to have what they call a second round and are preparing for it."