The Fairfax County Board of Supervisors voted yesterday to allow the nation's largest for-profit hospital chain to build a 127-bed hospital in Reston, an action that will end the Fairfax Hospital Association's monopoly on hospital service in the county.

The decision came a week after the board approved a similar zoning change to allow the nonprofit association to build its own hospital in the county's fast-growing western end.

Some board members say the two decisions have set the stage for an intense battle for health-care customers in a county where existing hospitals already are underused.

A spokesman for the Hospital Corp. of America, a for-profit group that operates more than 300 hospitals nationwide, said it plans to open the new Reston hospital in July of 1986. The FHA, which runs Fairfax County's three other hospitals, plans to open a 160-bed facility in the Fair Oaks area near Rte. 50 and Interstate Rte. 66 several months earlier.

Board members unanimously approved rezoning the Reston hospital site, despite reservations that the facility will not be fully used.

"This is not a certificate of need" for a hospital, said Centreville Supervisor Martha V. Pennino, who long has promised to bring a hospital to the Reston area. "We can all address that negatively. What we have here is strictly a land use issue."

Some of the bitterest fighting over both hospitals went on at the state level when the Virginia health commissioner first rejected both proposals, saying Fairfax County could not support more hospitals. He later reneged and approved both facilities, contingent on the closing of the private Circle Terrace Hospital in Alexandria and the association's Commonwealth Hospital in Fairfax City.

While the Fairfax supervisors lack formal control over whether the hospitals are needed, they hold the critical power over the rezonings that were needed for the two buildings, each estimated to cost about $30 million.

Pennino attempted to block approval of the land use changes for the association hospital last week. Although she based her opposition on inadequate transportation in the Fair Oaks area, some colleagues privately accused her of trying to create a more favorable climate for the Reston hospital.

While yesterday's board action ended some of that fighting, officials say the two hospitals will be competing vigorously for patients.

Northern Virginia's existing hospitals, reflecting a nationwide trend, have been suffering from decreasing occupancy. In the first six months of 1984, the average occupancy rate in the area was about 73 percent.

The state health commissioner has forbidden the for-profit Hospital Corp. of America to provide maternity and obstetrics services, traditionally one of the most profitable divisions, in its Reston hospital.

Citing the fast-growing population of young couples in the Reston area, Clair A. Schwob, a spokesman for the Hospital Corp., said the firm is planning to contest that ruling.

Fairfax County officials said the existing Reston Access emergency medical facility, operated by the hospital association, will remain open after the new hospital begins operation.