The judges have ordered the state to create a “low-risk list” of candidates for early release, but the state has yet to do so and the court has repeatedly chided the state. If good-behavior time credits do not produce enough releases by the end of 2013, the judges said the state will have to release “low risk” offenders, which critics say will produce a spike in crime.

The judges cited the state’s “repeated failure to take necessary steps to remedy the constitutional violations” in their latest ruling and said that any appeal of the plan would not push back Thursday’s order. The state must report its steps to reduce the population to the court every two weeks or be held in contempt, the judges wrote.

“The governor is supposed to act immediately,” said Donald Specter of the Prison Law Office, which initially sued the state over prison conditions in 1990 and argued in front of the Supreme Court. “As the court made clear, there should not be any significant impact on the public’s safety.”

But in a statement on Thursday afternoon, the governor made it clear he had no intention of releasing prisoners swiftly. “The state will seek an immediate stay of this unprecedented order to release almost 10,000 inmates by the end of this year,” Mr. Brown said.