WASHINGTON - The Supreme Court, its new chief justice joining the majority, made clear yesterday that juries, not judges, must determine facts that justify harsher prison sentences.

In a 6-3 ruling, the court struck down California's Determinate Sentencing Law, the latest in a series of decisions limiting judges' discretion in sentencing.

Thousands of California inmates may be eligible to have their sentences reduced, in many cases by about a year.

"This court has repeatedly held that, under the Sixth Amendment, any fact that exposes a defendant to a greater potential sentence must be found by the jury, not a judge, and established beyond a reasonable doubt, not merely by a preponderance of the evidence," Justice Ruth Bader Ginsburg wrote for the court.

The majority was one vote larger than the 5-4 rulings that have been the norm in sentencing cases. In his first major case dealing with the constitutionality of prison terms, Chief Justice John Roberts joined Ginsburg's majority opinion.

By contrast, the other new justice, former federal prosecutor Samuel Alito, issued a strong dissent. California's law "is indistinguishable in any constitutionally significant respect" from the federal sentencing guidelines that have been approved by the Supreme Court.

Justices Stephen G. Breyer and Anthony M. Kennedy also dissented.

Several states have changed their sentencing laws to require prosecutors to prove to a jury aggravating factors that could lead to longer sentences. The court did not prescribe a way to fix the California law. "The ball lies in California's court," Ginsburg said.

Justices Antonin Scalia, David H. Souter, John Paul Stevens and Clarence Thomas also were in the majority on an issue that confounds the typical conservative-liberal split on the court.

The ruling yesterday in Cunningham v. California could shave four years off the 16-year sentence of John Cunningham, a former Richmond, Calif., police officer. He was convicted of sexually abusing his 10-year-old son after the boy moved in with Cunningham and his girlfriend.

California had argued that a 2005 state Supreme Court decision interpreting the state law effectively brought the state into compliance with the U.S. high court's rulings. The law instructs judges to sentence inmates to the middle of three options, unless factors exist that justify the shorter or longer prison term. The judge in Cunningham's case imposed the longest possible term.

To read the Supreme Court ruling, go to http://go.philly.