The heads of the Montgomery County school board and County Council said yesterday they want to file a lawsuit to prevent the county government from leasing a vacant school building to a private school in an area that school officials say will interfere with public school integration.

Board president Blair Ewing and council president David Scull said yesterday legal grounds exist to stop operation of the Grace Episcopal Day School in the former Larchmont Elementary School building, closed three years ago. They have directed attorneys to look into the possibility, they said.

The decision to lease Larchmont to Grace Episcopal has caused a major flap among school officials, County Council members and NAACP members who appealed unsuccessfully last week to County Executive Charles W. Gilchrist to break his contract with the school.

In separate interviews yesterday, Scull and Ewing contended that opening the private school will exacerbate problems of declining enrollment in nearby public schools.

The Larchmont building -- for grades three through six -- is about a half a mile from North Chevy Chase Elementary, one of three elementary schools involved in a busing plan between neighborhoods that are predominantly white and predominantly black.

Grace Episcopal, which operates a school in Silver Spring, has an enrollment that is about one-third minority students.

Grace Episcopal officials have said they will limit the number of new students at the Larchmont building to 20 for the next three years in an effort to defuse any effect on the integration plan.

Gilchrist aide Ed Rovner said yesterday Gilchrist has not "seen anything that would change his mind" about the lease.

But, in an apparent softening from last week, Rovner said Gilchrist would be "more than amenable" to considering new possibilities.

Council members have drafted a letter saying serious constitutional questions can be raised about the lease's effect on the integration plan.

Scull said he expects the County Council to approve next week a resolution calling on the state agency that oversees public school buildings to block the lease.

Ewing said he plans to ask his fellow school board members to join him in asking the council to consider legislation that would prohibit the county from renting any vacant school building to a private school.