A Prince William Circuit Court judge had invalidated the 1973 rezoning of a 513-acre farmland tract near Manassaa that the Marriott Corp. planned to transform into a "Great America" theme park.

The ruling by Judge Arthur W. Sinclair appears to reduce further the chance that the theme park will be built in Prince William County and to increase the likehood that Marriott eventually will put the park in Howard County, Md.

Marriott attorney Stephen A. West said yesterday, "We're just trying to decide right now what to do. We're analyzing the whole thing."

Sinclair last week upheld the contention of the Prince William League for the Protection of Natural Resources that the county Board of Supervisors violated state law in its rezoning of the land, which lies between Interstate Tre. 66 and U.S. Rte. 29-211 and abuts the Manassas National Battlefield Park.

The board rezoned the land 13 days after advertising its public hearing. Sinclair ruled that the law requires at least a 19-day period.

The present Board of Supervisors, which strongly supports the park, may decide today whether to appeal the decision or repeat the rezoning process.

Board Vice Chairman Donald L. White said the board could wait for Marriott to reapply for the zoning chage or could move to rezone the land on the Board's own motion.

"It was the Board's intent that that land be developed, with or without Marriott," White said.

County Attorney John F. Rick said an appeal could take six months, while rezoning might require only three months.

Rick said Sinclair's ruling would take effect in about 10 days, as soon as a court order can be drafted and signed by the judge.

The League for the Protection of Natural Resources, an environmental group, has fought the Disney World-type park on grounds that it would overload the county's road and sewer systems, lead to unplanned development ringing the park and damage the integrity of the Manassas battlefield.

"The problem with Great America has always been that Marriott picked the poorest site in the county. We have a planned 7,000-acre industrial area and that's where they should have gone," said League President Memory Porter. She said she expects Marriott to seek rezoning of the land, but future actions by the league have not been decided.

Supervisor James Byrd, a Great American supporter, said, "That's the kind of industry we need in Prince William County and we need it bad." He and other supporters argue that the park wuld create jobs and expand the country's tax base.

The theme park would cost about $75 million to build and could add $26 million a year to a county's economy, according to Marriott estimates.

Marriott, while refusing to rule out, the Prince William site, has arranged to buy 230 acres near Columbia, between Interstate Rte. 95 and U.S. Rte. 29 if it can win Howard County approval for its park.

Marriott sought another Howard site for the park in 1972, hoping to take advantage of the interstate and the heavy population living in Baltimore-Washington corridor, but was rejected by the county. The firm then shifted to Prince William but was delayed by a faltering economy, legal battles and the decision to get other new parks near San Francisco and Chicago open and running before tackling a third one in the Washington area.

A company spokesman said Howard is now the preferred spot for a Washington area park and that Marriott has filed preliminary plans for the development there.

Howard County is currently drafting a new comprehensive zoning plan for the county, and the necessary rezoning for the park could be included in the new plan, subject to public hearings and County Council action.

One serious problem facing Marriott and Howard County could be the handling of sewage generated by a projected 2.6 million park visitors a year. The proposed site is currently under a sewer moratorium, according to county officials.

Marriott's lawyer West said yesterday that the park would not open before 1980, by which time sewer capacity should be available.

West said the Prince William site remains a valuable property because of its location at the I-66 interchange for Manassas and that he expects it will eventually be rezoned and developed.