WASHINGTON - The Supreme Court struck down California's sentencing law today, a decision that could mean shorter sentences for thousands of state prisoners.

The 6-3 ruling in Cunningham v. California effectively shaves four years off the 16-year sentence of a former police officer who was convicted of sexually abusing his son.

It's the latest in a series of high court rulings over the past seven years that limits judges' discretion in sentencing defendants. The court has held repeatedly that a judge may not increase a defendant's sentence based on factors that were not determined by a jury.

``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.

Several states have changed their sentencing laws to require prosecutors to prove to a jury aggravating factors that could lead to longer sentences.

California had argued that a 2005 state Supreme Court decision interpreting the state's Determinate Sentencing 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 state warned that its criminal justice system would be burdened by having to re-sentence thousands of inmates.