The Prince William County Board of Supervisors, in a rebuff to the Fairfax County supervisors, refused yesterday to impose mandatory water restrictions on Prince William residents.

The 6-to-0 vote was a serious blow to efforts to fashion a coordinated response to the Northern Virginia water shortage that has been created by the steady decline of the Occoquan Reservoir.

The reservoir, which supplies about 600,000 people in Fairfax County. Alexandria and Prince William, now contains about 3.3 billion gallons, the level at which criteria adopted by the Fairfax County Water Authority call for mandatory restrictions.

Fairfax already has approved mandatory conservation and was hoping to implement restrictions - which curtail outdoor uses - as early as Wednesday,

Despite Prince William's decision not to impose restrictions. Fairfax officials said the county would probably order mandatory conservation if Alexandra went along.

In their unanimous vote, the Prince William supervisors called for an elected official from all three jurisdictions to meet with technical staff members "and agree to agree to crieria and adopt whatever (measures), voluntary or mandatory, are deemed appropriate."

Fairfax Board Chairman John F. Herrity who sat glum-faced in the audience during the Prince William deliberations, said later. "We stand ready, able and willing to meet with the Prince William Board or a representative at a mutually convenient time and place."

While their only vote was on the motion to have a regional meeting, the Prince William supervisors made it clear they were opposed to mandatory restrictions. Chairman Alice Humphries said voluntary conservation measures now being followed by many citizens have been successful.

When the jurisdictions began a voluntary conservation effort early last month, their leaders met in joint sessions to stress that efforts were being coordinated. But an undercurrent of politics and rivalries quickly swwept away the spirit of cooperation.

Prince William officials have expressed deep resentment toward both the Fairfax Water Authority, which operates the reservoir, and the Fairfax government.

Prince William and Alexandria have been put in the position of being in the middle of a fight between the water authority and the Fairfax County Board of Supervisors." Prince William Board Chairman Humphries said. "We're either going to have to duck or start shooting back."

Most of the shooting was done by Laurence E. Randall. Prince William's representative on the water authority board and an outspoken critic of mandatory restrictions at this time.

"We do have a water supply problem," Randall said. "We do not have a crisis . . . Mandatory rationing is not required. It would put a blemish on the county in trying to attrack industrial development."

Chairman Humphries permitted Randall, who is not a member of the county staff, to make the only persentation heard by the supervisors before they voted. In the long prepared statement he read. Randall said "some Fairfax County officials, without the knowledge of the water authority. Prince William County or the City of Alexandria, reached a decision to cause a regional imposition of mandatory water restrictions. . ."