California law against slaughtering and selling the meat of sick and injured animals has been struck down by US Supreme Court.
The Supreme Court said California's law ran afoul of the Federal Meat Inspection Act.
The California law forbids a slaughterhouse to "buy, sell, or receive a nonambulatory animal," butcher it or sell its meat, or hold it without immediately euthanizing it.
Federal law has no requirement of immediately euthanizing the animals.
The California State Legislature passed the law in response to a documentary released in January 2008. It showed obviously sick animals just before being slaughtered and cruel treatment of them by slaughterhouse workers at two plants in Chino, California.
The film showed the animals being dragged with chains, rammed with a forklift or having pressurized water squirted up their nostrils to get them to move.
However, the Supreme Court unanimously ruled that California lacks authority to make regulations different from federal law at slaughterhouses inspected by the US Food and Drug Administration.
"The California law runs smack into the (federal) regulations," Justice Elena Kagan wrote for the Supreme Court.
The lawsuit reached the Supreme Court after pork processing companies sued to overturn the California law.
The US Ninth Circuit Court of Appeals in San Francisco upheld the state law but the Supreme Court reversed the decision.
The key issue in the case was a provision of the 1906 Federal Meat Inspection Act that forbids state regulations of slaughterhouses that are "in addition to, or different than those made under" the Federal Meat Inspection Act.
Some of the meat from US slaughterhouses is destined for foreign markets, where the impact of the Supreme Court decision is uncertain.
Joe Schuele, spokesman for the US Meat Export Federation, said the California law was unnecessary.
"We have a downer law that is effective in keeping sick and infected animals out of the food chain," Schuele told AFP.
The cruelty and lax safety practices displayed by the California meatpacking plant workers in the documentary showed "they were in violation of the law," Schuele said. "It wasn't the lack of a law that led to that problem."
The United States exported about $6 billion in pork and $5.3 billion in beef in 2011, according to the US Department of Agriculture.