Britain's opposition Labor Party, struggling to recover from its worst election defeat in half a century last May, split down the middle today in an ideological battle at its annual party conference.

The party's insurgent left wing, led from behind the scenes by former energy secretary Anthony Wedgwood Benn, and its slipping right wing, led by former prime minister James Callaghan, are fighting each other for control of the party and over how far it should go in building a socialist society in Britain.

Callaghan today beat back an attempt by the left wing to change the method by which the party's leader is chosen -- a change that would enable left-wing leaders to oust Callaghan from the job.

They blame him and his centrist philosphy for Labor's lopsided loss to Prime Minister Margaret Thatcher's Conservatives in the May national election.

But the leftists won approval of a proposal to force all Labor members of Parliament to submit regularly to reselection by their local Labor Party constituency committee. This could mean the removal of a dozen Labor members of Parliament in districts where party committees are much more leftist than their legislators.

The left wing also is expected to win approval tomorrow of a proposal that would shift control of the party's election platform from the leader and members of Parliament to the much more leftist Executive Committee. Before last May's election, Callaghan eliminated from the platform some of the left wing's most radical proposals.

In this and other ways, the party's leftist chairman, Frank Allaun, charged at yesterday's conference session at the English Channel resort of Brighton, Callaghan lost the May election by ignoring the party's nonparliamentary leaders and its grass-roots' supporters.

"Whatever we say," Allaun said of Callaghan and the party's leading members of Parliament, "they take no notice."

With the defeat of many leftist Labor parliamentary candidates in the Conservative landslide last May, the Labor legislators are generally more conservative than the party's nonparliamentary national and local leadership. They do not want to extend the scope of Britain's welfare state, introduced by the Labor Party, or to increase public ownership of industry.

Leaders of the party's left wing believe that a grass-roots crusade for socialism is necessary to defeat Thatcher's conservative crusade against socialism. But Callaghan and his supporters in the parliamentary wing of the party fear that, if Labor becomes too radical or fights too much internally, it will never regain control of the government.

Pleading unsuccessfully today for conference delegates to defeat all three left-wing proposals for change, Callaghan warned that the party "should consider very carefully before it fractionalizes into 635 pieces."

This week's party conference at Brighton, perhaps the most bitter and divisive in Labor's history, only marks the beginning of formal fighting in a war that has been building for some time. There is now to be a year-long inquiry, conducted by an ideologically split party commission, into how the Labor Party should be restructured for the future. It will report to next year's party conference, where even fiercer battles are expected.

Although other members of the party's left wing have mounted the attack on Callaghan at Brighton this week, their silent leader has been Benn, a pipe-smoking intellectual politician who recently said he would be a contender for party leader if and when Callaghan stepped down.

Callaghan, who wants to retire soon, hopes to pass the leadership on to one of his centrist supporters.The burly former chancellor of the Exchequer, Denis Healey, is the senior and leading centrist contender.

If Labor's members of Parliament continue to choose the party leader, a centrist favored by Callaghan would be likely to win. But Benn would have a much better chance if the selection process were instead dominated by Labor's left-wing party leadership, as was proposed in Brighton.