Under increasing pressure to move prisoners out of Prince William County's old and overcrowded jail, the Manassas City Council has approved its share of a three-month operating budget to open a new, $5 million regional detention center that will serve both the city and county.

In recent weeks, while awaiting completion of the new facility, 131 inmates have been crowded into the 32-year-old county jail in Manassas, which was designed to house 30 prisoners.

A U.S. District Court jury in Alexandria ruled last Friday that Prince William County had held about 7,000 prisoners in unconstitutionally unsafe, overcrowded conditions between August 1980 and January of this year. The Virginia American Civil Liberties Union had initiated the suit after two prisoners complained that poor sanitary conditions had caused skin disease and eye infections to spread among prisoners forced to sleep on mattresses on the floor.

Within 30 days, a hearing will be scheduled to set damages in the case, U.S. District Judge Richard L. Williams said.

"We don't want to be subjected to that type of action in the future," Manassas City Manager C. M. Moyer said. "We want to move ahead with the new jail."

The Manassas City Council agreed last week to pay 20 percent of the new jail's $779,376 operating budget for the next three months, with Prince William County picking up the remainder of the costs. Council members next Monday are to receive a report from Moyer proposing that the city's share of the operating budget for the year starting July 1 be $151,000.

Although the total operating budget for the facility is $3.4 million for the coming year, the state is expected to pay $1.7 million of that cost. The remainder will be shared between the county and Manassas.