Prince William County, bitterly opposed to a new multimillion-dollar sewage treatment plant built on Bull Run, balked yesterday and said it would refuse to hook into the system as scheduled later this month.

The action could force a legal battle over use of the Upper Occoquan Sewage Authority's plant, the most advanced sewage treatment facility in the United States, now set to begin serving Prince William County June 26.

The $82 million regional sewage plant is designed to replace 11 smaller facilities. Discharge from the older plants has been polluting the Occoquan Reservoir, the source of drinking water for 600,000 Northern Virginians.

Officals of the regional authority -- built to serve Fairfax and Prince William counties and cities of Manassas and Manassas Park -- refused to comment on the action yesterday.

But David S. Bailey, Virginia State Water Control Board enforcement director, said, "We have made plans for this. We have met with our attorneys ... Those people who are required to hook up will be required to hook up."

The regional plant's "usefulness is not going to be jeopardized by people pulling out at the last moment. I think we have adequate legal grounds to ensure that that doesn't a happen," Bailey said.

THe reaction was prompted by a unanimous vote yesterday by the Prince William County Board of Supervisors to send a letter to the authority stating that late June was "premature" for the county to agree to use the expensive and controversial facility.

County Attorney Terence Emerson refused to comment on whether the county intended to flight in court, repeating that the county's position wa only that the agreement was "premature."

Emerson cited language in the UOSA agreement stating that jurisdictions are not obligated to participate unless the federal and state governments pay 80 percent of the cost, which was originally estimated at $42 million.

He also cited a provision that construction must be "economical and efficient." Emerson said the county was not satisfied that those requirements had been met.

He also said the hook up should not be made until metering devices have been installed at two connections between Prince William County and Manassas.

Delays in construction, changes in plans and inflation pushed the cost of the plant -- which one county supervisor has called "a gold-lined chamber pot" -- rapidly upward. Prince William officials now contend that sewer and water bills will triple for county residents connected with UOSA. Manassas Park officials say bills there will quadruple.

In Fairfax County, which will spread the costs over the entire jurisdiction, bill have gone from 95 cents fpr 1,000 gallons to $1.41. In Manassas, rates will go from $1.46 to $1.60 for 1,000 gallons.