A judge on Thursday ruled against Gov. Arnold Schwarzenegger's furlough order for thousands of California prison guards in a decision that could cost the state millions.

Alameda County Superior Court Judge Frank Roesch sided with the 30,000-member California Correctional Peace Officers Association and ordered the state to pay prison guards back for the days they worked without pay.

The union argued that Schwarzenegger's furlough order amounted to an illegal wage cut because prison guards could not take time off fast enough due to the nature of their work.

"We are thankful for the judge's ruling regarding our peace officer members receiving compensation for the time they worked," said CCPOA spokesman Lance Corcoran.

Corcoran said it's not clear how much the decision could cost the state but estimated it to be in the millions.

Schwarzenegger's spokeswoman Rachel Arrezola said the governor plans to appeal. The administration argued the furlough plan at the Corrections Department is working because it gives workers flexibility to schedule their time off without disrupting prison operations.