The Reagan administration has fostered widespread housing segregation by failing to enforce or strengthen fair housing laws that have been largely ineffective for a generation, a bipartisan committee of 15 former government officials charged yesterday.

The committee released an analysis of 1980 census data for 28 major cities that said that while segregation declined slightly in the last decade, particularly in the South, many northern cities remained as racially divided as in 1970.

The average "segregation index" dropped from 87 to 81 on a scale in which 100 means that every city block is totally white or black, and zero represents complete integration.

Chicago is the most segregated of the cities studied, with an index of 92, followed by Cleveland, 91; St. Louis, 90; Philadelphia, 88, and Baltimore, 86. Washington, 79, is in the middle range, and Oakland, at 59, is the least segregated city, according to the study by University of Wisconsin demographer Karl Taeuber. Segregation dropped by more than 10 points in Dallas, Houston, Jacksonville, Nashville and Richmond.

On a second index, the study said that 67 percent of the white residents in Cleveland and St. Louis live on blocks that have no blacks, as do 64 percent of the whites in Philadelphia and 59 percent of the whites in Chicago.

The citizens' committee, headed by Arthur S. Flemming, former chairman of the U.S. Civil Rights Commission, criticized the administration on a range of housing issues in the 116-page report:

The Justice Department has filed only five housing bias lawsuits during the current administration, compared with an average of 32 a year previously. Justice has indicated that it will not pursue exclusionary zoning cases and is concentrating on minor suits that will not set legal precedents, the report says.

* The Housing and Urban Development Department still has no regulations to implement the 1968 Fair Housing Act. Reagan officials withdrew regulations proposed in 1980 and have not resubmitted them.

* HUD's fair housing office spends more than a third of its enforcement budget on community boards to convince realtors to obey the law voluntarily. These boards cannot file discrimination suits and are forbidden from publishing the names of realtor members.

* HUD is no longer building subsidized apartments for the poor and is pushing a "voucher" program for tenants that will do nothing to alleviate a widespread shortage of low-income housing.

"In the field of housing, our nation continues to relate to minorities in an unconstitutional, illegal and basically unfair manner," Flemming told reporters. "We're getting anything but vigorous enforcement of the Fair Housing Act at the present time."

While low-income housing programs have been sharply reduced, the panel said, the government continues to spend billions of dollars a year on housing tax subsidies and mortgage interest deductions for the middle class.

Flemming said several members of Congress soon will introduce legislation to strengthen fair housing laws, in part by allowing HUD to process bias complaints by administrative action.

The committee includes former labor secretary Ray Marshall, former attorney general Elliot L. Richardson, former senator Birch Bayh and the Rev. Theodore M. Hesburgh, president of Notre Dame University.