The Reagan administration, in another shift in civil rights enforcement, has adopted a largely voluntary approach toward landlords and brokers who discriminate against people looking for housing.

At the Housing and Urban Development Department, whose authority in such cases always has been limited, officials now stress cooperation with the real estate industry rather than contributing to aggressive civil rights groups as in the past. They have created, with much fanfare, a network of housing advisory boards, but the boards are not permitted to investigate fair-housing complaints or to sue anyone for discrimination.

An even more dramatic change has occurred at the Justice Department, which wields the government's only real legal power in these cases. Justice has filed only two housing bias lawsuits in the two years since President Reagan took office, compared with a previous average of more than 30 cases a year.

Knowledgeable officials say that Justice lawyers have been told to stop bringing these cases unless they can prove intentional discrimination, a more rigorous standard than the courts require. Even Antonio Monroig, HUD's assistant secretary for fair housing, says he believes this is too high a legal hurdle.

Monroig said he doesn't understand why Justice has accepted only two of the 25 housing cases referred by HUD last year.

"I would certainly like to know their criteria for filing the suits so we don't waste our time sending a case over that's not going to be filed," he said.

In general, however, Monroig argues that working with the real estate industry is the best way to eliminate the causes of discrimination.

"We are giving more emphasis to the voluntary programs, but we're not diminishing enforcement," Monroig said. "We'll never be able to take every complaint to court. We don't have the resources, and it would be very costly. These voluntary programs try to change people's attitudes instead."

But critics are skeptical.

"The effectiveness of HUD's fair housing office has never been at as low an ebb as it is now," said Martin E. Sloane, director of the National Committee Against Discrimination in Housing. "Morale there is so low as to be indescribable."

The office is providing $2 million this year, nearly half its grant money, to finance 90 community housing resource boards that are supposed to bring industry officials and fair-housing advocates together "to discuss their mutual problems."

Last fall, at a conference of board officials, HUD Secretary Samuel R. Pierce Jr. praised the industry members for signing a new voluntary agreement to obey the fair-housing laws. In many of their communities, however, the agents' names remain a secret. The National Association of Realtors is afraid that they would be singled out for "testing" by civil rights groups, so HUD has promised not to reveal their names.

"It's a disgrace," Sloane said. "How can they take leadership in furthering fair housing if you don't know who they are and can't find out?"

But Sloane and others say the boards are useless anyway because they are barred from sending people to test whether landlords and brokers are discriminating. Fair-housing advocates also are required to resign from the board if their group sues another industry member.

Bill North, a spokesman for the Realtors group, said its members are free to announce their agreements with HUD if they wish.

"Their identities are made known on a need-to-know basis," North said. "It isn't really intended for public use." While some of the community boards are effective, he said, certain fair-housing advocates have taken a confrontational approach toward his members. "They feel like they exist to be law-enforcement officers," he said.

Others say the real law-enforcement officers are no longer doing their job. A recent study by the Washington Council of Lawyers, a nonprofit legal group, said the Justice Department's fair-housing effort "has deteriorated dramatically . . . .The present administration has retreated from almost 15 years of vigorous commitment to fair housing."

The study criticized William Bradford Reynolds, assistant attorney general for civil rights, for no longer filing suits against those whose actions had the "effect" of discriminating. Justice officials say Reynolds also has given a low priority to challenges to local zoning laws that exclude minorities.

"The lawyers have been ordered not to bring cases unless they can prove intent to discriminate," said Robert Reinstein, the division's litigation chief during the Carter years. "I think it's pretty clear they're opposed to bringing the big cases. Many of the attorneys there have a sense of futility."

The Civil Rights Division, in a recent rebuttal, described its record as "most impressive." The division said that 59 housing cases are now under investigation, eight have been settled by consent decree and five are being prepared for trial. It also said that challenging only the "effects" of discrimination "would play havoc" with legitimate zoning laws.

A Justice spokesman said the division is concentrating on cases that would open large apartment complexes to minorities. But Reinstein and other critics say the two suits filed so far break no new legal ground and already had been filed by community groups.

In one case, a Boston fair-housing group, Education Instruccion, filed a complaint with HUD in 1980 against a large realty firm. HUD tried without success to arrange a voluntary settlement for 18 months.

HUD has never had much success in settling such complaints. Last year, the department received 3,710 new complaints and reached settlements in 674, some held over from the previous year. More than 3,200 cases eventually were dropped.

"The message that went out to the real estate industry was that you can thumb your nose at HUD," said Pat Morse, the Boston group's director. The group later went to court and Justice brought a suit last March.

The settlement reached by Justice requires the firm to advertise in black-oriented media and to keep a record of all applicants, but it imposed no fine and doesn't give Morse's group access to the records.

"The rug was pulled out from under us," Morse said. "If a big real estate company pays no penalty, there's absolutely no deterrent to discrimination."

"They've given lip service to fair housing, but they've cut out programs and cut out staff," said Sterling Tucker, who preceded Monroig in the HUD post.

HUD also has killed a set of stricter bias rules that took Tucker's office two years to write. Instead, Monroig said the agency will ask Congress to strengthen its powers under the 1968 Fair Housing Act. The last effort to beef up the law died in 1980 after a Senate filibuster.