In a 6-2 decision, the court upheld Delaware's right to block plans for a $600 million plant on the New Jersey side of the Delaware River.

The proposed plant in Logan Township, Gloucester County, would have had a pier extending 2,000 feet into the river to unload ships carrying natural gas that had been cooled to a liquid.

The problem for New Jersey is that Delaware's border extends to the low-tide mark on the New Jersey shore within a 12-mile circle centered on New Castle. That unusual configuration dates to the 1682 land grant to William Penn.

British energy giant BP PLC proposed the gas plant, known as Crown Landing, in 2003 and sought a permit for the pier from Delaware. In 2005, the state rejected the request because unloading natural gas at the site would have violated a Delaware law that limits industrial activity along the coast.

New Jersey then took the fight to the U.S. Supreme Court.

Writing for the majority, Justice Ruth Bader Ginsburg said Delaware may not impede ordinary projects on the New Jersey shore. "The Crown Landing, however, goes well beyond the ordinary or usual," Ginsburg wrote.

John A. Hughes, secretary of the Delaware Department of Natural Resources and Environmental Control, said the ruling showed that Delaware had the law on its side.

"I'm certainly not trying to rub New Jersey's nose in it," Hughes said. But his agency could not have approved a permit for the pier "without gutting the Coastal Zone Act," he said.

Leland Moore, a spokesman for New Jersey's Office of the Attorney General, which filed the lawsuit, said: "We are disappointed the U.S. Supreme Court did not uphold our position that New Jersey has exclusive jurisdiction to regulate all improvements emanating from our shore."

Tom Mueller, BP's government and public affairs director, said the company would not give up.

"This is an important energy project for the nation and for the region. We will continue to explore other options and anticipate that the project will move forward and eventually be built here on the Delaware River," he said.

Farther south, the border between the states is in the middle of the Delaware Bay, giving New Jersey more leeway for large projects.

Two Gloucester County Democratic state legislators, Assemblyman John Burzichelli and Sen. Stephen M. Sweeney, said yesterday's divided court decision "underscores the need for the two states to revisit the century-old compact that governs land use" along New Jersey's side of the Delaware River.