Left-wing militants seeking a socialist transformation of Britain and unilateral nuclear disarmament narrowly failed tonight to unseat the deputy leader of the opposition Labor Party.

In a contest that had become symbolic of deep and bitter divisions between the party's right and left wings, deputy leader Denis Healey defeated challenger Tony Benn with a little more than 50 percent of the votes cast under a new party electoral-college system.

Benn, 56, a former Cabinet minister, was the candidate of Labor's leftists, who want to nationalize much more of British industry, eliminate private education and health care, pull out of the European Community and ban nuclear weapons and American bases from Britain. After having succeeded in recent years in having most of this adopted as official policy by the party machinery, over which it has gained control, Labor's left has sought to make it binding on the party leadership in Parliament.

Healey, 64, also a former Cabinet minister, and other parliamentary leaders of the party have resisted these efforts, which they said were narrowing Labor's appeal to voters and depriving it of an opportunity to gain ground on the right-wing Conservative government of Prime Minister Margaret Thatcher during a time of economic crisis.

Healey's victory tonight at the beginning of Labor's annual party conference here was the first significant setback suffered by the leftists in their drive for power in the party.

Afterward, Healey said the vote showed that a majority of Labor supporters "wanted to reestablish the party as a responsible and broad-based movement of left, right and center which is tolerant of dissent and based on common sense."

Healey said he hoped there would now be fewer defectors in the future from Labor's ranks to the centrist political alliance of the new Social Democratic Party and the Liberal Party, which seek to capitalize on the increasing political polarization of Labor on the left and the Conservatives on the right.

Most of the members of Parliament and local government around Britain who have joined the Social Democratic Party since its founding earlier this year have been defectors from Labor unhappy with its recent movement to the left.

Earlier today, Healey and supporters, including former Labor prime minister James Callaghan, held a rally here that Callaghan called the start of a "fight-back" by the party's parliamentary leadership against the left-wing insurgents. "The challenge," Callaghan said, "is, will this party remain the great coalition it has always been or just become a narrow sect appealing to only one group of voters and give up its natural government majority?"

Left-wing activists supporting Benn, however, also vowed today to continue their efforts to force the party's leadership to carry out their more militantly socialist policies if Labor regains power in Britain.

One Benn supporter, member of Parliament Michael Meacher, said tonight's vote "leaves the party in the odd position where the deputy leader is in opposition to its key policies. We hope Mr. Healey will recognize the support for these policies demonstrated by the support we have won."

The Labor Party was founded early this century as the political voice of the labor union movement. It launched Britain's welfare state after World War II and while it often has been divided between right and left factions, the current split is the party's deepest.

Those on its right, usually a majority among Labor members of Parliament, have worked primarily to expand state welfare programs in a mixed economy of private and government business ownership. Those on the left, usually strongest among party activists, have wanted a Labor government to be more militantly socialist. They also have long sought to ban nuclear weapons in Britain, even if it meant leaving the NATO alliance.

Party leftists accuse the last Labor government, led by Callaghan from 1974 to 1979, of making possible Thatcher's election victory in 1979 by veering too far from the party's socialist policies and, in the words of one leader of the insurgents, mineworkers' union official Arthur Scargill, "trying to practice capitalism better than the Conservatives."

Callaghan retired from the party leadership in 1979 and was replaced by Michael Foot, the compromise choice between Benn and Healey. The left then won two important rules changes designed to make future Labor governments more responsive to the party organization.

Local constituency groups were given more power to replace Labor incumbents as election candidates when they fail to follow party policy. Selection of the party's leader and deputy, who would be prime minister and his deputy if Labor returned to power, was removed from Labor members of Parliament and given to the electoral college, in which the members of Parliament have only a minority voice. The other components of the college are constituency and union groups.

The electoral college was used for the first time in the deputy leadership contest between Benn and Healey. The vote and public opinion polls show that Benn and the leftists were supported by most constituency and union activists, while Healey retained the support of a majority of Laborites in Parliament, and politically inactive rank-and-file unionists and party members.

Ironically, Healey's small margin of victory was provided by about two dozen Labor members of Parliament who support the new leftist policies but oppose the attempts by Benn and the left-wing activists to dictate them to the parliamentary leadership. After supporting a third candidate for the deputy leadership, John Silkin, on tonight's first ballot, they abstained on the second.