Britain's deeply divided opposition Labor Party was thrown into further disarray today when it voted by narrow majorities to change the way the party's leader and candidates for Parliament are to be chosen in the future.

The changes, made at the party's annual conference, would give a much greater voice in both selection processes to leftists who have taken over many grass-roots constituency groups as well as the party's national executive. They want the next Labor leader and the party's members of Parliament to offer a more militantly socialist alternative to the free-market policies of Conservative Prime Minister Margaret Thatcher. Today's votes at the conference in Blackpool were a major victory for former energy minister Anthony Wedgwood Benn, the outspoken leader of the leftists. The group seeks to expand Britain's welfare state, nationalize more of its industry, abolish all private schools and private health care, take the country out of the European Common Market and ban NATO nuclear weapons from British soil.

The decisions were a setback for the party's current leader, former prime minister James Callaghan, who has tried to keep the party on a more centrist course, working for a mixed economy, continued British participation in the Common Market and NATO, and nuclear disarmament only by multilateral East-West agreement.

Callaghan, 68, a patriarchal figure, has been expected to retire later this year. His successor was to be chosen by the party's elected members of Parliament. The front-runner was former chancellor of the exchequer Denis Healey, an aggressive, sometimes abrasive defender of these centrist policies.

But one of the party conference votes won by the left wing today would assign the selection of the next leader to an electoral college that also would include representatives of the grass-roots constituency groups and the labor unions could lead to the party's financial support. This could lead to the selection of someone more acceptable to both the left and right wings, rather than Healey.

However, the party was unable to agree today on the precise makeup of an electoral college.If if fails to do so before the conference ends this week, the leadership selection would be left to the party's members of Parliament long enough for Healey to succeed Callaghan -- assuming he retires.

The left wing also won a narrow conference vote to force all labor members of Parliament to face periodic mandatory reselection by their local constituency parties to be certain they continued to reflect the voter's views.

The left narrowly lost a third vote, however, on allowing the party's national executive -- now controlled by the left wing -- to take away from the parliamentary leadership the final say on the platform that future Labor governments would be pledged to carry out.

Benn and others of the party's left wing, including some national union leaders, argued that changes were necessary to make the party more democratic by returning control of policies to the grass roots.

Other former Labor government ministers and union leaders answered that the left wing was being undemocratic by trying to force its radical socialist policies on the party.

The most outspoken of Benn's opponents from the right wing of the party, former education minister Shirley Williams, warned of "a fascism of the left." She urged an informal meeting of party conference delegates to fight or "you won't have a Labor Party worthy of the name."

She and others from Labor's right wing have threatened to create a new party if the left wing succeeds in committing Labor to policies they could not support.

Callaghan made an emotional speech yesterday urging unity in the face of Thatcher's survival-of-the-fittest economic policies. "For pity's sake, stop arguing," Callaghan told the party. "The public is crying out for unity in order to get rid of the Thatcher government."