SAN FRANCISCO — A federal appeals court on Tuesday banned the Justice Department from prosecuting medical marijuana cases if no state laws were broken.

A three-judge panel of the 9th US Circuit Court of Appeals ordered the federal agency to show that 10 pending cases in California and Washington state violated medical marijuana laws in those states before continuing with prosecutions.

Marijuana remains illegal under federal law, but Congress has barred the Justice Department from spending money to prevent states from regulating the use or sale of medical pot.

Federal prosecutors argued Congress meant only to bar the department from taking legal action against states and that it could still prosecute individuals. The court rejected that, saying medical marijuana prosecutions prevent states from implementing their own measures.

Federal prosecutors could ask the 9th Circuit to reconsider or petition the US Supreme Court to take up the issue. The Justice Department said officials are reviewing the decision.

Marijuana activists and lawyers representing medical pot suppliers said the ruling is a significant addition to the growing support for broad legalization of the drug. Marijuana is legal for medicinal or recreational use in 25 states and the District of Columbia. Ten states have marijuana legalization measures on the November ballots.

‘‘This is the beginning of the end of federal prosecutions of state medical marijuana dispensary operators, growers and patients,’’ said Marc Zilversmit, an attorney representing five people who operate four marijuana stores in Los Angeles and nine indoor growing sites in Los Angeles and San Francisco.

Yet Zilversmit and other medical marijuana supporters said the Obama administration and federal authorities are still fighting the drug’s legalization.

On Thursday, the Obama administration said marijuana will remain on the list of most dangerous drugs, but it will allow research into medical uses.