The Supreme Court on Monday blocked a California law that would require euthanizing downed livestock at federally inspected slaughterhouses to keep the meat out of the nation's food system.

The high court ruled that the state's 2009 state law was blocked from going into effect by federal law administered by the Agriculture Department's Food Safety and Inspection Service. .

Federal law "precludes California's effort ... to impose new rules, beyond any the FSIS has chosen to adopt, on what a slaughterhouse must do with a pig that becomes non-ambulatory during the production process," said Justice Elena Kagan, who wrote the court's unanimous opinion.

California strengthened regulations against slaughtering so-called "downer" animals after the 2008 release of an undercover Humane Society video showing workers abusing cows at a Southern California slaughterhouse. Under California law, the ban on buying, selling and slaughter of "downer" cattle also extends to pigs, sheep and goats.

But pork producers sued to stop the law, saying the new law interfered with federal laws that require inspections of downed livestock before determining whether they can be used for meat.