Taking its biggest step yet toward forcing a historic political realignment in Britain, the new centrist alliance of the upstart Social Democratic Party and the perennially also-ran Liberal Party won its first parliamentary election by a large margin yesterday in the south London suburb of Croydon.

The alliance-backed candidate, William Pitt, won 40 percent of the vote in the constituency to defeat John Butterfill of the ruling Conservative Party, who had 30.5 percent, and Stanley Boden of the Labor Party, with 26 percent. The rest of the votes were scattered among nine fringe candidates.

Pitt's easy victory demonstrated the attraction of the Social Democratic-Liberal alliance as an alternative to the right-wing economic policies of Prime Minister Margaret Thatcher's Conservatives and the leftward drift of the opposition Labor Party. Pitt, a bearded, bespectacled local government housing administrator, had never come close to winning the parliamentary seat in three previous tries as a Liberal and had gained only 10.5 percent of the vote in 1979.

"With this vote, we have split the old party system wide open," Pitt said early this morning. "The alliance has caught the imagination of the voters. Our momentum is unstoppable now."

His defeat of Butterfill, a prosperous real estate appraiser, in a constituency held by the Tories since 1948 is a significant political setback for Thatcher. The Conservative vote fell considerably from a victorious 49.4 percent there in 1979.

This could embolden a growing number of senior Conservative critics of Thatcher, including some members of her Cabinet, who have warned that her monetarist policies and harsh rhetoric are worsening Britain's severe economic crisis and eroding the Conservatives' political base.

Cabinet dissidents are pressing Thatcher to increase government spending and cut interest rates to reduce record high unemployment. Conservative rebels are threatening to put up a challenger against Thatcher as party leader next month, in the hopes they could attract enough secret ballot votes to force her to change course -- even though they believe they would have little chance of defeating her.

The third-place showing of Boden, a personable high school history teacher, is an even worse blow for Labor. Instead of increasing its vote as the major opposition party would be expected to do midway through the five-year term of an increasingly unpopular government, Labor lost ground from its 40.1 percent share of the vote in Croydon Northwest in 1979.

Labor's economic spokesman, Peter Shore, blamed ideological turmoil in the party caused by insurgent left-wingers who wanted to offer a more militantly socialist alternative to Thatcher.

"The lesson for us," Shore said, "is that we have to behave in a much different way."

Yesterday's election focused intense attention on Croydon, a predominantly middle-class area of modest Edwardian row houses and shop-lined commercial streets.

The Liberals and Social Democrats smothered the constituency with several thousand party workers from across the country campaigning for Pitt. Among them were Liberal leader David Steel and the four former Labor Cabinet members who lead the Social Democrats -- Roy Jenkins, Shirley Williams, David Owen and William Rodgers.

Labor leader Michael Foot and former prime minister James Callaghan came to speak for Boden, while several Thatcher Cabinet members and former prime minister Edward Heath campaigned for Butterfill.

Pitt's victory gives the new centrist alliance 12 Liberal and 21 Social Democratic members of Parliament, compared to 336 Conservatives, 248 Labor members and 16 from minor regional parties. The other 11 Liberals were elected in the last national election in 1979, while 20 of the Social Democrats defected from the Labor Party and one from the Conservatives, all during the past year.

The next parliamentary test for the new alliance is expected this winter in the suburban constituendy of Crosby near Liverpool in northwest England, which has been particularly hard hit by Britain's worse recession in a half century. Williams, one of the Social Democrats' leaders and a former education secretary, will contest the Crosby seat, recently vacated by the death of a Conservative who won 57 percent of the vote in 1979.

Under the electoral agreement between the Social Democrats and Liberals, they are alternating candidates in by-elections and will divide the country's constituencies roughly in half in the next national election.

Pitt's 40 percent share of the Croydon northwest vote approximately matches the level of support the Social Democratic-Liberal alliance has received in national opinion polls and in local government by-elections scattered throughout Britain in recent months. This could be enough for it to win control of parliament in the next national election in 1983 or 1984.

A number of voters leaving the polls in Croydon told reporters and pollsters they switched to Pitt because of their dissatisfaction with the Conservatives and Labor. The question to be answered by future parliamentary elections is whether this is a temporary desertion to register a protest or a more permanent shift in response to the increased polarization of the Conservatives and Labor in a country that has appeared traditionally to prefer centrist government.

"In British politics, it is the center ground that is vital. The center ground is where elections are won," said Norman St. John Stevas, former Conservative leader of the House of Commons, whom Thatcher fired from her Cabinet because of his opposition to her policies. He was among many senior Conseratives at the party's annual conference last week to warn that the Social Democratic-Liberal alliance could capitalize on the Conservatives' identification with Thatcher and Labor's adoption of policies promising more nationalizations, prohibition of private education and health care, withdrawal from the European Common Market and unilateral nuclear disarmament.

"The British people will not vote for an extremist party," St. John Stevas said at the Conservatives' conference. "At the very moment when the Labor Party is turning itself into an extremist organization, we have contrived to make ourselves appear as though we are marching to a similar dead end."

The Social Democrats and Liberals favor maintaining Britain's mixed economy of both government and private business ownership, staying in the Common Market and working for multilateral disarmament. Their most radical proposal is to change Britain's voting system to proportional representation, which would be likely to further erode the power of the Conservatives and Labor and favor coalition rather than one-party governments.