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.

"The governor does not believe that state workers should be shielded from the same economic realities that every California family and business is facing," Arrezola said.

Schwarzenegger had imposed three mandatory unpaid days off each month for more than 200,000 state workers this year to help close a multibillion-dollar budget deficit. In March, the administration won a ruling from Sacramento County Judge Patrick Marlette allowing the governor to furlough union and nonunion employees in a state budget crisis.

That decision is being appealed.

In Alameda County, the prison guards union successfully argued that because their members worked at 24-hour facilities, they would lose pay because they could not take time off fast enough. Many other state workers who work at state business offices have been taking three Fridays off each month.

The deadline for redeeming furlough time is June 2012.

Roesch's decision applied only to prison guards. Unions representing some other state workers have separate lawsuits before the judge.

In a related action, the prison guards union also filed a federal suit in U.S. District Court in San Francisco asking the court to declare that the state is violating federal wage laws.

California is facing another deficit — $21 billion — in the new fiscal year starting July 1, 2010.