At least 9,000 U.S. Postal Service jobs will become union positions over the next three months due to an arbitration ruling this month that said post offices must use bargaining-unit employees at sites with reduced operating hours.

Arbitrator Stephen B. Goldberg said in his decision that the postal service violated an agreement with the American Postal Workers Union by using part-time employees instead of union clerks at many of the lower-traffic facilities where the financially struggling agency reduced hours in 2012 to save money.

The ruling requires the postal service to use union clerks at the facilities that have cut back to four hours or six hours per day, but it allows the agency to continue using non-union employees at sites that have been reduced to two hours per day.

The American Postal Workers Union, which filed the grievance against the USPS, said the decision requires six-hour offices to establish 3,000 full-time, bargaining-unit positions, while the four-hour offices must establish 6,000 union-backed jobs for part-time workers known as postal-support employees.

The reduction in hours at low-traffic post offices came as a compromise after labor groups and members of Congress pushed back against earlier plans to close the facilities entirely.

Prior to the arbitration ruling, the postal service used part-time postmasters and “postmaster reliefs” to fill the positions. Less than 350 jobs were held by union clerks.

The APWU filed a grievance in 2012 and agreed to enter arbitration in January, hoping to resolve a legal dispute that had already lasted nearly two years.

“This is historic,” said APWU official Bob Johnson, who assisted the union in its case. “We haven’t had APWU members in most of these offices in decades, and now to have full-time positions in six-hour offices — that’s phenomenal.”

The postal service took the decision in stride Tuesday. USPS Chief Human Resources Officer and Executive Vice President Jeffrey Williamson said it “gives us some flexibility in staffing across administrative post office groups.”