A strike Tuesday by port clerks has shut down the largest cargo terminal at the Port of Los Angeles and threatens to spread to all port terminals in Los Angeles and Long Beach pending a ruling by a local labor arbitrator.

Port clerks working at APM Terminals, which operates Pier 400 at the Los Angeles port, went on strike at noon. The clerks are members of the International Longshore and Warehouse Union’s Local 63 Office and Clerical Unit, or OCU, which represents about 800 office clerks at local port terminals. Longshoremen at Pier 400 have honored the OCU’s picket line, shutting down the terminal.

OCU representatives and attorneys for the Los Angeles/Long Beach Harbor Employers Association, which represents terminal operators, were meeting Tuesday afternoon with a labor arbitrator to determine if the OCU’s strike is valid and if longshore workers are allowed to honor the picket. If they are, it could lead to a harbor-wide strike that would shut down all cargo terminals in San Pedro and Long Beach.

A ruling was expected later Tuesday, but it appears likely the arbitrator could sign off on the strike. An arbitrator ruled in April that longshore workers should have been allowed to join a previous OCU strike.

Representatives for the union could not immediately be reached for comment. Steve Getzug, a representative for the employers association, decline to comment beyond confirming the hearing.

The OCU and harbor employers have been negotiating for more than two years for wages, benefits and job security.