The former leader of the House of Commons, Michael Foot, an eloquent parliamentary orator and crusader for nuclear disarmament, was chosen tonight to be the new leader of Britain's ideologically divided opposition Labor Party by a 10-vote majority of Labor members of Parliament.

Foot, supported by the party's left-wing proponents of a more radically socialist alternative to the right-wing Conservative government of Prime Minister Margaret Thatcher, beat the right-wing candidate, former chancellor of the Exchequer Denis Healey, by 139 to 129 in a runoff vote by Labor members of Parliament.

Foot, 67, a frail-looking former journalist and political maverick with a gentle manner but a powerful presence as a public speaker, steps up from deputy leader to replace former prime minister James Callaghan, who retired as head of the party last month at the age of 68.

With a surprisingly large margin of victory in the secret balloting, Foot must have won votes in the tight contest from centrist members of Parliament who felt he was more able than the sometimes brusque and pugnacious Healey to heal the party's deep divisions.

Foot's supporters, including influential union leaders, predicted tonight that he would have little difficulty winning ratification, if necessary, in a new leadership selection by a proposed Labor Party electoral college next year. Its composition of representatives of Labor legislators, trade unions and local party constitutuency groups is to be decided by a party conference in January.

Former energy secretary Tony Benn, leader of left-wing insurgents who have captured control of the party's national organization and a majority of its constituency groups, already has declared that he will be a candidate for the party leadership in the new electoral college. Benn, 55, passed up the leadership selection conducted by Labor members of Parliament, arguing that it was no longer valid after delegates at the party's annual conference last month decided by a narrow majority to set up an electoral college but failed to agree on its composition.

The left-wing insurgents want to greatly expand the welfare state, outlaw private schools and health care, abolish the House of Lords, nationalize many more industries, restrict competition from imports, withdraw from the European Community, and ban nuclear weapons from Britain.

Foot does not necessarily subscribe to all these views, although he believes strongly in arms control and the welfare state. He opposed Britain's entry into the Common Market and supports abolition of the House of Lords.

He has promised to try to pull the party together and launch an immediate campaign "to transform the winter of fear and desolation ahead of us into a hurricane of national protest" against Thatcher's tough monetarist economic policies.

He also vowed to press for nuclear disarmament by Britain. He said he would work to reverse Thatcher's decision to modernize Britain's nuclear deterrent with new U.S.-made Trident submarine-launched missiles and her agreement to upgrade NATO nuclear weapons based in Britain with U.S. cruise missiles. He wants Britain to lead the West in ending "the most dangerous arms race in history."

His principal task, however, will be keeping both the party's left and right wings together under the umbrella of his leadership. He is expected to have the support of all but the most militant left-wingers so long as he remains leader.

Healey, the leadership candidate of the right-wing proponents of a mixed economy, strong defense and continued British participation in NATO and the Common Market, offered tonight to serve as Foot's deputy. This makes it less likely that many prominent right-wing Labor legislators would bolt the party unless the new leadership embraced all of the left wing's most radical policies.

A fiery left-wing orator during three decades as an iconoclastic parliamentary backbencher, Foot also was a staunch protector of parliamentary tradition.

After finally joining the party leadership in 1974, becoming employment minister and then leader of Commons, Foot became best known for helping engineer pragmatic political compromises that kept a minority Labor government in power until Thatcher's election victory last year. He submerged his own left-wing views to serve Callaghan.

Foot worked particularly closely with labor union leaders while in the Cabinet, sometimes being accused by political opponents of being their captive. His labor leader friends are believed to have pushed the reluctant Foot into the party leadership contest as a unifying candidate they could count on being able to work with.

Foot still faces concern about his age and image as potential prime minister when Labor next has the opportunity to challenge Thatcher's conservatives at the polls, which is not expected much before 1984. Foot, the oldest man elected Labor leader in a half century, would then be 70.

He walks with the aid of a cane since a serious auto accident in 1963. But he remains vigorous and exudes energy on the stump, his white hair flowing and his thick, horn-rimmed glasses slipping down his nose as cuts opponents with his wit.

Born into a middle-class south-western England family with seven children, Foot grew up with books and politics.

He attended an exclusive Quaker private school and Oxford University, where he became president of the Oxford Union, Britain's most prestigious student debating society. He worked as a journalist for Lord Beaverbrook, a legendary British press tycoon with whom the young Foot developed an unlikely close relationship.

He is married to an author, Jill Craigie and lives in Hampstead, a fashionable neighborhood for writers and artists. Long known as an unaffected intellectual with long hair and rumpled dress, Foot cut his hair short and put on a new suit for parliamentary appearances during the short party leadership campaign.