BRIGHTON, ENGLAND, SEPT. 29 -- Labor Party leader Neil Kinnock called on his party today to learn the lessons of a third straight electoral defeat by modernizing its policies to attract more voters.

Calling for a "rigorous reassessment . . . across the whole field of our policies," Kinnock made clear in his keynote speech to the annual party conference that neither Labor's proposed unilateral nuclear disarmament, nor its constitutional commitment to state ownership of industry were immune from review.

Kinnock told left-wing activists to subdue their criticisms and join the party mainstream, or risk further narrowing Labor's appeal.

From now on, Kinnock said, all party efforts must be undertaken "in the clear and certain knowledge that we address many people who need to be convinced if they are to make the shift to supporting us, people who need to be certain that their trust in our common purpose and common sense is fully justified."

The conference is the first party gathering since June's general election brought massive victory to Conservative Prime Minister Margaret Thatcher and a continuation of her hefty parliamentary majority. Some pundits are predicting that the party will never again win power in Britain.

Part of Labor's problem has been that the bedrock of its support, Britain's traditional working class, is shrinking, with employment decreasing in heavy industry while it increases in service and high-tech industries. But the party's left has found it hard to argue with electoral figures showing that even in the traditional working class, less than half of all semiskilled and unskilled workers, and an even smaller percentage of trade union members, voted for Labor.

Many of those workers cite benefits from Thatcher's programs allowing them to purchase the public housing units where they live, and offering some of the country's largest state-owned enterprises for sale -- even to buyers with the money to buy only a single share.

Labor politicians campaigning during the June election found many of these new capitalists feared Labor's belief in the public provision of housing, and its constitutional commitment to public ownership of the nation's "means of production."

According to numerous Labor strategists, the party must change not only the substance, but even the terminology, of its policies in this area. "Even the word 'socialism' sounds old-fashioned these days," said one. "Words like 'collectivism,' if they mean anything to most people, mean something negative."

At the same time, Labor seemed unable during the campaign to capitalize on its strengths -- including the widespread belief of many voters that Thatcher is "uncaring" about things like unemployment and the continuation of the National Health Service.

Although Labor was perceived as the "caring" party across a wide spectrum of voters, many did not believe that the party, widely perceived as disorganized and fractious, was capable of putting its commitment to the collective good into efficient practice.

In the view of the party leadership, that image has been enhanced in recent years by the so-called "looney left," the primarily local officials whose concerns about minorities have led them into extremism that has alienated middle- and lower-class voters.

This week's conference was carefully planned by the leadership to capitalize on the membership's despair over the election, and Kinnock's own sustained popularity, to inaugurate organizational and policy changes.

In votes during yesterday's opening sessions, Kinnock won changes limiting the power of far-left activists to select parliamentary candidates. The selection of new members of the party's ruling National Executive Council increased the existing pro-Kinnock majority.

Bryan Gould, the party's rising pro-Kinnock star, was returned to the council, but he polled fewer votes than London activist Ken Livingstone -- seen as an outspoken darling of the left.

A third vote agreed to a no-holds-barred review of party policies over the coming year. Kinnock pledged that "we are not going to jettison our commitments" to socialism and collective provision of basic services. "Nothing would more deserve the charge of cynicism or sabotage credibility than to make a bonfire of everything we stood for" during the last election, he said.

But the party, he said, had to face the "social realities" that included increased home ownership, earlier retirement and "many more people owning shares," alongside continuing problems of poverty and unemployment.

"Democratic socialism," he said, "has got to be as attractive and as beckoning and as useful to the relatively affluent and secure as it is to the less fortunate in our society who are referred to as our 'natural vote.'

"Any citizen in our society who has got the franchise is a citizen to whom we could and should be able to make an appeal."

On the subject of nuclear weapons, Kinnock congratulated the Reagan administration and the Soviet leadership for their progress toward arms control. He accused Thatcher, who has said she now wants the arms control process to slow down, of trying to "block" progress.

Labor strategists say they are hoping that a new agreement removing intermediate-range nuclear weapons from Europe, including U.S. cruise missiles based in Britain, will eliminate the need for them to call for their removal when this country next goes to the polls.

In the past, Labor has been accused of endangering nuclear arms negotiations by offering to give up Britain's independent nuclear deterrent unilaterally. Today, Kinnock said only that the policy review would "work to ensure that we have policies that are capable of dealing with the changed conditions of the 1990s in a way that will enhance the prospect of removing reliance on nuclear weapons."