Britain's ideologically torn opposition Labor Party moved this week to the brink of a formal split that could bring about the first significant realignment of British political parties in 60 years.

Thirteen present and former Labor members of Parliment led by four former Cabinet ministers took the first step toward forming a breakaway social democratic party if the Labor Party's leaders do not reverse its recent sharp turn to the left.

The splinter group -- headed by Roy Jenkins, former deputy Labor leader and European Community president, former foreign secretary David Owen, former education secretary Shirley Williams and former transportation secretary and opposition defense spokesman William Rodgers -- this week formed a Council for Social Democracy to be the nucleus for a new party that could come in the spring.

Nine other Labor members of Parliament immediately joined the group and another former Labor deputy leader, Lord George Brown, said he would back it with a grass-roots dissident group he heads.

They will now seek public support as a left-of-center alternative to what they see as a polarization of British politics between Prime Minister Margaret Thatcher's Conservatives on the right and a militantly socialist and isolationist Labor Party on the left under the struggling leadership of parliamentary orator Michael Foot.

The Labor dissidents oppose both Thatcher's survival-of-fittest economic policies, which have greatly increased unemployment and the gap between rich and poor, and the greater government control of the economy, including much more nationalization of industry, sought by left-wing insurgents who have seized at least temporary control of the Labor Party. Labor's left wing, led by former energy minister Tony Benn, also wants to pull out of the Common Market and unilaterally ban nuclear weapons from Britain.

In a left-wing victory seen as the final straw by the dissidents, a special party conference decided Jan. 24 that future party leaders should be chosen by an electoral college with 40 percent of the voting power held by labor unions that finance the party, 30 percent by mostly left-wing local party activists and 30 percent by the generally more moderate Labor members of Parliament, who traditionally elected the party leader by themselves.

Foot, who has been fighting what appears to be a losing battle to keep the party together since his compromise election as leader in November, failed in an attempt to preserve for members of Parliament 50 percent of the votes.

In launching the breakaway Council for Social Democracy this week, Jenkins, Owen, Williams and Rodgers condemned what they called "the drift toward extremism in the Labor Party."

"We believe that the need for a realignment of British politics must now be faced," they said in a statement launching the movement.

They want to pursue policies more like those of social democratic parties elsewhere in Europe. These include staying in the Common Market and NATO, working for multilateral arms control and maintaining a mixed economy with private ownership of most businesses but possibly government price and income controls.

They differ from the Liberal Party, already struggling to stay alive in Britain's political center (with 11 seats in the 635-seat House of Commons), primarily in their espousal of greater social equality through more aggressive redistribution of income and the breaking down of class barriers in schools and other institutions. The Liberals, once Britain's traditional laissez-faire party, were replaced by Labor as one of the two major parties following a political shift that began in the 1920s and ended with the postwar birth of Britain's welfare state.

But the leaders of the new Social Democratic movement here and Liberal leader David Steel are near agreement on a cooperative election strategy to divide the country's parliamentary districts, with each party fielding its strongest candidates without competition from the other.

Steel, trying to sell this plan to both traditional Labor Party supporters and skeptical Liberal voters and members of Parliament who see it as a threat to their own party, said this week that it could create "an unstoppable combination at the next general election."

"This is the chance to do what many of us have dreamed of for years," Steel said in a nationwide political broadcast, "to break the mold of a failed political system and to produce a realignment of the progressive and hopeful forces in Britain."

Recent public opinion polls have shown growing interest among at least a third of Britain's voters in a liberal-social democratic alternative to the Conservative and Labor parties. But cautious political analysts note the strong tradition of party loyalty here and the absence of a single dominating leader for the prospective centrist alternative. A breakaway social democratic party could, however, siphon sufficient support from Labor to ensure Thatcher's Conservatives at least a decade in government despite the current unpopularity of her harsh economic policies.

While testing the political water, the dissidents are still participating in the Labor Party, which provoked angry responses from both Foot and Benn at a meeting of the party executive Wednesday attended by Williams and another dissident member of Parliament, Tom Bradley.

"If they are plotting to form a new party," Benn was reported by others there to have shouted, "they cannot sit here in the highest councils of the Labor Party with all the access to party documents available to them."

"You had better make your mind up what you want to do," Foot is reliably reported to have sternly told Williams. "If you want to join another party, it is quite intolerable that you should sit here."