Twelve members of the British Parliament, led by two former Cabinet ministers, broke away from the ideologically split opposition Labor Party yesterday Party in the House of Commons.

Joined by a number of other defectors from the Labor Party, which has taken a sharp turn to the left since losing the last parliamentary election two years ago, they announced plans to crete by mid-April a formal party organization for Britain's first grouping in more tha a half century.

The Social Democrats join the Liberal Party in the center of the British political spectrum, between the more militantly socialist and isolationist Labor Party on the left and Prime Minister Margaret Thatcher's ruling Conservative Party on the right.

Some of the Social Democrats and Liberal leader David Steel hope to make an electora alliance to try to greatly increase their combined strength in the next national election.

Although they differ on many specific issues, both the Liberals and the Social Democrats portray themselves as defenders of Britain's postwar mixed economy and welfare state against Thatcher's survival-of-the-fittest free market policies and Labor's declared determination to increase government ownership of business and control of the economy.

At a time when Britain is suffering its worst recession since the 1930s and its politics have become increasingly polarized, the emergence of the new party and its possible aliance with the Liberals has had a much greater impact than would be expected from their numbers in Parliament -- 12 seats for the Social Democrats and 11 for the Liberals against 337 for the Conservatives and now 255 for Labor.

More than 40 percent of the British voters say they would support a liberal-Social Democratic alliance insted of Labor on the Conservatives, according to public opinion polls in Britain. Polical analysts there doubt, however, whether voters will depart that much from their strongly traditional balloting pattern in the next parliamentary election in 1983 or 1984.

But the new party and the Liberals could win enough votes and seats away from the others to deny Thatcher's Conservatives a parliamentary majority or dramatically accelerate the slow decline of the Labor Party, whose share of the vote has shrunk steadily during the last several national elections. This could eventually reverse the political shift in Britain earlier this century in which the Liberals were replaced by Labor as one of the two major parties.

Before the next national election, the Social Democrats must decide on a single party leader and a distinct political program. They also must decide whether to accept Steel's offer to campaign together on jointly agreed general principles and not contest each other's areas of greatest strength.

Four former Labor Cabinet ministers now lead the new party: former foreign secretary David Owen and former trasportation secretary and defense spokesman William Rodgers -- who are among the 12 with seats in Commons -- and former education secretary Shirley Williams and former deputy Labor leader Roy Jenkins, who are not now in Parliament. Williams is the popular choice for party leader in opinion polls, but she would first have to win a parliamentary seat when one became vacant.

Nine Labor members of the House of Lords, including three more former government ministers, also broke away and formed a new Social Decomcratic grouping in the less powerful, predominantly hereditary upper chamber of Parliament.

The Social Democrats generally oppose Thatcher's tight-money, deflationary economic policies and the unemployment these measures have helped casue. They favor instead spending much of Britain's burgeoning North Sea oil revenue to expand the economy and create jobs. In answer to Thatcher's contention that this would increase inflation, which she has succeeded in sharply reducing, they argue that the government should impose wage and price controls.

The Social Democrats also oppose the widespread nationalization of industry and finance advocated by the insurgent left wing of the Labor Party, led by former energy minister Tony Benn. In foreign policy, they oppose the aim of both the leftists and Labor leader Michael Foot to withdraw from the European Community and to ban nuclear weapons from Britain, although they strongly favor East-West arms control negotiations.

Leaders of the Social Democrats said they were finally pushed out of the Labor Party by recent rules changes giving the leftist-controlled party machinery at both the national and constituency level much more influence in the choosing of the party leader, selecting parliamentary candidates, second-guessing sitting Labor deputies and deciding party policy.

Political analysts in London believe the Social Democrats chose today to declare their parliamentary independence so they could present their own position in House of Commons debate Tuesday on Thatcher's plans to modernize Britain's independent nuclear deterrent, Polaris submarine-launched missiles, with American Trident submarines and new british-made nuclear warheads.

Now seated separately from the Labor Party on the opposition benches in the House of Commons, the Social Democrats also are expected to respond independently to the presentation of Thatcher govrnment's annual budget next week. Their economic philosophy most nearly resembles that of social democratic parties elsewhere in northern Europe, where there is less government ownership of business but more generally financed welfare state programs than in Britain.

Other Labor moderates denounced the split yesterday as "divisive and defeatist." But Owen said at a news conference in London, "We are on the receiving end of a political earthquake . . . . We are wholly unique in British politics.