Hundreds of truckers who haul freight from the California ports of Los Angeles and Long Beach launched a strike Monday against four ground-shipment companies — a move that could revive labor tensions at the nation’s busiest cargo hub.

The outcome of the dispute has implications for hundreds of companies and thousands of truckers in Southern California serving the ports, which handle 43 percent of containerized goods entering the United States.

Delegations of drivers notified the companies of their intent to strike at 6 a.m., with picket lines going up immediately at the companies' truck yards, said Teamsters spokeswoman Barb Maynard. The strikers also plan to picket marine terminals, rail yards and other locations where the companies dispatch trucks.

The companies affected by the strike are Pacific 9 Transportation, Intermodal Bridge Transport, Pacer Cartage and Harbor Rail Transport.

Another company, Green Fleet Systems, was hit by previous port truck strikes; the Teamsters are not including the company in this strike because the two parties have agreed to "a comprehensive labor peace agreement," said a statement put out by Green Fleet and the Teamsters on Monday. As part of the agreement, Green Fleet Systems will allow its drivers to choose whether to join a union.

The truckers at the other companies accuse them of carrying out wage theft by illegally misclassifying drivers as independent contractors instead of employees with the right to union representation. Pacific 9 driver Amador Rojas, speaking in a media conference call through a translator, complained that drivers are expected to pay out of pocket for truck maintenance.

Pacific 9's requirement that drivers pay for their own truck maintenance "is one of the issues causing distress not just for us but for our families as well," he said.

About 500 port truckers have filed wage claims with state labor officials accusing their companies of misclassifying them as freelancers and charging them to lease the trucks they drive.

Thousands more drivers have yet to file claims, and national port-trucking companies could be liable for wage and hour violations of up to nearly $1.4 billion annually, the labor-backed National Employment Law Project has estimated.

As part of the labor action beginning on Monday, the drivers are asking the mayors of Los Angeles and Long Beach to meet with the truck companies to discuss working conditions.

In January truckers won a $2 million judgment against Pacer Cartage in a misclassification suit supporters say could bolster class-action litigation against other companies. But Pacer has said it would appeal the decision.

It was not immediately clear how disruptive the union's action will be. A series of similar strikes last year caused little disarray at the ports.

The action comes as West Coast port cargo traffic has returned to normal after months of slowdowns over a dispute between shipping companies and the International Longshore and Warehouse Union. That dispute was resolved in February with a five-year labor accord.

Al Jazeera and Reuters