President Reagan's appointee as chairman of the board of the Legal Services Corp. has proposed new regulations that would virtually prohibit government-funded poverty lawyers from filing class-action lawsuits against government agencies on behalf of poor people.

Legal Services lawyers have brought successful class-action suits against prison systems for overcrowding, mental institutions for poor living conditions and public housing programs for segregation. A Texas legal services program was also responsible for the case decided by the Supreme Court this year allowing the children of illegal immigrants to attend public schools in Texas.

The Reagan administration has argued that the federal government has no business using taxpayers' money to tell states how to run their institutions.

Ellen Vargyas, an attorney with the National Legal Aid and Defender Association, called the proposed rules a "blatant violation of the [Legal Services] act.

"Insofar as the board is concerned with following the law, they will reject these regulations," she said.

David Landau of the American Civil Liberties Union added, "There is no lawsuit over prison conditions, housing violations or conditions in mental institutions that could be brought under those rules. The people in those institutions are indigent. They don't have money. There's no one else to help them. They will not be able to bring the lawsuits and the conditions will go unremedied."

When Reagan took office in 1981, he asked Congress to abolish Legal Services, the semi-independent agency that provides free legal assistance to poor people, but Congress refused to go along. The president has since nominated a new 11-member board that shares much of his philosophy.

The proposed rule change, published last week in the Federal Register, would stipulate that any class action against the "federal government or any state or local government or government-type organization" may not be brought if it could lead to a court order requiring increased government expenditures.

In addition, the new rules would require Legal Services lawyers to obtain "consent" from every member of the class before the suit can be brought, a stipulation that Legal Services lawyers called "impossible."

Class-action suits, which have been used often in the past by poverty lawyers to force the federal, state and local governments to improve conditions in various institutions and programs, have been attacked repeatedly by conservatives.

The rules change, which is being pushed by board chairman William F. Harvey, a law professor at the University of Indiana, is accompanied in the Federal Register by a much more moderate proposal submitted by the Legal Services staff.

The staff proposal, which sets up general rules for class-action suits, was discussed by the board last month, but in an internal memorandum Harvey called the staff recommendation "very deficient." He charged it did not comply with the intention of Congress, which had asked the corporation to adopt "policies and regulations" regarding class-action suits.

In the board meeting, Harvey insisted that both the staff recommendation and his own be published. The board is expected to choose between the two in December after a period for public comment.

Up to now, Harvey has had a solid majority on the board supporting his actions. On Oct. 29, the board voted 8 to 2 to choose as its president Donald Bogard, a former law student of Harvey's who had earlier been rejected by the board's presidential search committee.

Harvey could not be reached yesterday for comment.

Meanwhile, the previous board of Legal Services has filed a lawsuit challenging the validity of the current board. If that suit is successful, it could raise questions not only about the validity of Bogard's appointment, but also about any rule changes adopted by the present board.

Most of the board was appointed while Congress was in recess. The lawsuit, which was rejected at the district court level and has been appealed, calls the recess appointments illegal.