On his final day in office last year, the Education Department's top civil rights official issued a sweeping policy statement that sharply curtailed the agency's ability to enforce antidiscrimination laws in school districts.

The new policy, outlined in an internal memorandum obtained by The Washington Post, reversed a longstanding practice of threatening to cut off federal funds to force compliance with civil rights laws.

The memorandum was issued Dec. 30 by Harry M. Singleton, then assistant education secretary for civil rights. It directed regional civil rights directors to pursue cases only when there is discrimination in the individual project or class -- not the entire school -- that receives part of a federal block grant.

Singleton said that if a school uses its share of a block grant to teach the metric system in a mathematics class, for example, only that class and its teachers are subject to federal antidiscrimination enforcement.

The policy extended to block grants the 1984 Supreme Court ruling in Grove City College v. Bell, which held that federal aid intended for a specific program requires civil rights compliance only in that program.

About 15,000 school districts nationwide receive federal block grants that can be used for a variety of programs, and the department therefore required an entire district to refrain from discriminating or risk losing its grant -- even after Grove City.

Civil rights advocates criticized the new policy yesterday as effectively eliminating the department's civil rights enforcement power while making it easier for school districts to discriminate. Some predicted that the policy, which affects hundreds of pending cases, might backfire and add impetus to a move on Capitol Hill to restore the enforcement powers gutted by the Grove City ruling, handed down two years ago today.

"This action may be the single most devastating blow to civil rights yet made by the Reagan administration," said Rep. Ted Weiss (D-N.Y.), chairman of the House intergovernmental relations subcommittee that oversees the Education Department's civil rights office. "It will eliminate civil rights protection for millions of students."

"It's ludicrous," said Phyllis McClure, director of the NAACP Legal Defense and Education Fund. "It trivializes civil rights enforcement because they [federal officials] now have to trace every single dollar down to some infinitesimally small unit just to see if they have jurisdiction."

In a letter accompanying the memorandum, Singleton said the new policy was an outgrowth of an October 1985 ruling by a departmental review board, which said that a school district in Pickens County, S.C., could continue to discriminate against girls in gym classes because none of its federal block grant was used for physical education.

The board said it based its decision on Grove City. The Supreme Court said that if part of an institution receiving federal funds is found to discriminate, the federal government can cut off funds only to that part of the institution.

Singleton could have appealed the review board's decision in the South Carolina case. Instead, he issued the memo making it department policy.

A department spokesman said: "Congress is considering legislation to reverse the Supreme Court's Grove City decision. [Meanwhile] we are obligated to follow Supreme Court interpretations of the law."