Last year's Falklands War erupted as an emotional new issue in Britain's general election campaign today, after Denis Healey, deputy leader of the opposition Labor Party, accused Prime Minister Margaret Thatcher of having "gloried in the slaughter" caused by the conflict.

Thatcher responded by accusing Healey of going "beyond all bounds of public or political decency." Other leading Conservatives and spokesmen for the Alliance of Liberals and Social Democrats also denounced Healey's attack and suggested that Thatcher's reelection chances may be helped by it.

Widespread indignation over Healey's remarks only adds to Labor's already considerable troubles. A series of polls published this week shows an increase for the Alliance mostly at Labor's expense. Although still trailing the Tories by a wide margin, the Alliance could erode some of Labor's strongholds.

Until now, opposition politicians had hardly mentioned the Falklands War during the campaign nor had the government. All sides seemed to recognize that Thatcher's successful handling of the war had boasted her popularity to record levels, a lead over her rivals she still maintains. Although the prime minister has been criticized for her refusal to consider any negotiations with Argentina over the islands' future, her attitude toward the war had never been challenged.

Last night Healey abandoned the taboo in a speech clearly intended to jolt listeners.

Healey said Thatcher was "wrapping herself in a Union Jack and exploiting the services of our soldiers, sailors and airmen and hoping to get away with it." He said that Thatcher, "who glories in slaughter," was seeking to take advantage of the "superb professionalism of our armed forces." He repeated the slaughter remark today.

In a television debate today, however, Healey, noting he had chosen his words badly, said he should have used the word "conflict" instead of "slaughter."

In a separate attack yesterday, Healey's Labor colleague, Neil Kinnock, demanded that Thatcher defend her decision to order the sinking of the Argentine cruiser, General Belgrano, in which 368 Argentines were killed. Kinnock said only a public inquiry would allay suspicions that Thatcher ordered the move to upset peace mediationthen under way.

Undercurrents of doubt about the Belgrano incident have persisted amid rumors, originating mainly in South America, that Thatcher knew the cruiser was returning to port on May 2, 1982 when the order to sink was conveyed by the prime minister. At the time, the ship was just outside Britain's "total exclusion zone" around the islands.

Thatcher's position has been that the Belgrano had shifted course many times and eventually would have engaged British warships. "The allegations that have been made are totally false," Thatcher said again today. Healey's remarks, however, aroused the greatest controversy. As Labor's spokesman on foreign affairs, Healey is also the party's most popular senior figure, according to recent polls. He is well known for his combative style, which until today was considered one of Labor's biggest assets in its uphill effort to topple Thatcher's Conservatives.

At a news conference this morning, Healy was unrepentant about his charges. "If it is slanderous, then she'll sue me," he said. He said Thatcher had been "extraordinarily feckless and remiss" in not preventing the war from taking place.

Healey also assailed Thatcher's refusal to block large-scale lending by British banks to the Argentine government, which he said, could be used "to buy weapons, including weapons made in Britian to kill British servicemen." This, he said, "is nauseating hypocrisy."

Thatcher did not reply specifically today to Healey's remarks on the actions of banks. British banks have resumed commercial relations with the Argentines since the end of the fighting with the British government's approval. But Thatcher did say his comments had "given offense to many, many people in this country."

David Owen, a former Labor foreign minister who is now deputy leader of the Social Democrats, said Healey had blundered.. In the year since the Falklands conflict ended, he said, the public verdict on the handling of it had been rendered and was positive. "Healey will live to regret this," Owen declared.