Port truck drivers from three of Los Angeles’ leading drayage firms began indefinite strikes on Monday morning over claims of unfair labor practices at the twin ports of Los Angeles and Long Beach.

The action is the fourth such strike in the past 11 months and is an escalation from prior ones that were just 24 to 48 hours in duration, all with the backing on the Teamsters Union.

Around 120 drivers are reportedly involved in the strike targeting the trucking companies Green Fleet Systems, Total Transportation Services, and Pacific 9 Transportation.

The group organizing the strike, Justice for Port Drivers, claims Monday’s strike comes as the drayage industry is growing “increasingly desperate and retaliatory” to hold onto a business model that relies on independent contractors. It says multiple rulings by the California Division of Labor Standards Enforcement and Employment Development Department, along with the National Labor Relations Board, have determined these carriers should be classifying many port truck drivers as employees.

“Port truck drivers work long hours hauling nearly $4 billion worth of cargo every day, yet often receive paychecks below the minimum wage,” said Justice for Port Drivers in a release. “These professional drivers, who transport imports from American seaports for companies like Walmart, Home Depot, Skechers Shoes, and Polo/Ralph Lauren, are on the front lines of the fight for a fair day’s pay for a hard day’s work. Unwilling to wait for the lawless industry to transform, drivers are rising up, to improve their jobs and rebuild the ever diminishing middle-class.”

In June, the National Labor Relations Board charged one of these carriers, Green Fleet Systems, committed more than 50 labor law violations against its drivers.

It allegedly fired and threatened drivers in a number of different ways for union activities, retaliated against drivers who supported unions and planted an operative in its driver workforce to spy on drivers who supported union organizing.

Green Fleet Systems has denied the charges.