Hugh Gaitskell, leader of Britain's Labor Party, died Friday of complications arising from pleurisy. Many observers believed that Gaitskell, 56, would be the next prime minister.

"He was a man of great integrity, as well as a very brilliant man," said Samuel H. Beer, professor of Government. "He succeeded against great difficulties in pulling his party together."

Beer said that Gaitskell "was not a clever politician. His integrity was so strong that he couldn't always make adjustments, but in the long run he won everybody's respect. He had many friends in Washington and Cambridge."

The son of a colonial civil servant, Gaitskell decided early that "my future is with the working classes." He graduated from Oxford with honors in philosophy, politics, and economics, and began touring England's mine fields, lecturing on socialism. In 1934, he went to Vienna on a Rockefeller scholarship.

During World War II, Gaitskell left a university post to enter Britain's coalition government. Labor's landslide victory in the 1945 elections brought him to Parliament, where he rose to the position of Secretary of the Ministry of Fuel and Power.

Prime Minister Clement Atlee apopinted him Chancellor of the Exchequer in 1950, one year before the Conservatives returned to power. Gaitskell became head of the Labor Party in 1955, when Atlee was named an earl and joined the House of Lords.

Gaitskell's greatest victory as party chief came during the 1961 conference, when he quashed a movement for unilateral disarmament. At the preceding conference he had been forced to submit to strong "ban-the-bomb" sentiment.

Last autumn Gaitskell announced his opposition to British membership in the Common Market, and many thought his position was strengthened by de Gaulle's rejection of the British bid. Observers believe that Gaitskell will be succeeded either by his deputy, George Brown; Labor's export on foreign affairs, Harold Wilson; or its fiscal authority, James Callaghan.

Gaitskell delivered the Godkin lectures at Harvard in January, 1957. Speaking to capacity crowds in Sanders Theater, he urged the Great Powers "to give the app- essary leadership" in the United Nations, and advocated the admission at Red China and the eventual redistribution of votes on the basis of population.

In his second lecture, Gaitskell proposed the creation of a European neutral bloc including both Germany and the Soviet satellites. The neutrals' security, he said, would be "guaranteed by the Great Powers as well as by each other."

Gaitskell's final lecture called for United States co-operation in reaching a Suez settlement. The resignation of Prime Minister Anthony Eden interrupted Gaitskell's lecture series, but Gaitskell decided to remain in Cambridge and finish the speaking engagement