Walmart and two staffing agencies in the Chicago area were named as defendants in a class action lawsuit filed Monday in Illinois. The suit alleges that Walmart, Labor Ready Midwest, and QPS Employment Group violated federal and state laws by denying workers legally mandated overtime pay and in some cases, paying them less than minimum wage for hours worked.

The 20 initial plaintiffs in the case each worked for one of the two staffing agencies and were regularly placed at Walmart stores for temporary work, often stocking shelves in the stores. Walmart utilizes hundreds of such workers in the Chicago area alone, and plans to hire thousands more nationwide during the upcoming holiday season, according to a UFCW press release. Read the full complaint here.

Walmart spokesman Dan Fogleman told The Huffington Post that the company is still reviewing the complaint, but that "based on the UFCW's press release, one thing is clear. This litigation is being driven by the same union organizations that have been mischaracterizing several issues about Walmart and are more concerned with creating publicity than with improving workers rights.

"We are committed to ensuring that anyone working in our stores -- whether they're employed by Walmart or in this case, a temporary staffing agency -- is treated appropriately and compensated fairly for every hour they work," he added.

Anthony Wright, a temp worker at Labor Ready who is named in the suit said in a statement to the media, "There have been so many times I've been told to stay late after my shift to finish stocking the shelves, but I didn't know they wouldn't pay me for it." Wright has worked at some of the area Walmart stores since late last year, he said.

Another worker, Twanda Burk, said in a statement to the media, "I only get paid minimum wage and yet Labor Ready and Walmart still try to cheat me by not paying me for the time I actually work. I've proven that I'm a good worker, and they just want to take advantage of that."

The workers allege that Walmart failed to keep records of the time they worked, making it impossible for them to prove that they were underpaid by the staffing agency. They also allege that Walmart and the agencies failed to pay the minimum required amount to workers who were told to come in to work only to be sent home because they were not needed.

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The suit alleges that the violations stretch back to 2009 and continue to the present. The plaintiffs are seeking back pay and an injunction against the three companies. “The practices that Walmart and its staffing agencies are engaging in are exactly why the Illinois legislature passed the Illinois Day and Temporary Services Act,” said the workers' attorney in the case, Chris Williams of Workers’ Law Office. “Workers need critical information to make sure they don’t get cheated on their pay, as they did here. These workers are required to be paid for the time they’ve worked.”

The case comes as Walmart, the world's largest private employer, is facing a string of walkouts and labor-related charges in Illinois and other states. Earlier this month, Walmart workers at 28 locations nationwide protested alleged retaliation by management against workers who sought to organize their co-workers to protest for better treatment.

Walmart has a long history of union-busting, but a confidential Walmart management memo obtained by the Huffington Post earlier this month revealed that managers are being carefully instructed not to discipline striking workers in the current round of labor activities. The strikes are being organized by OUR Walmart, a coalition of current and former Walmart associates backed by the United Food and Commercial Workers Union. In addition to striking earlier this month, OUR Walmart is also formulating plans for concerted labor activities at Walmart stores on Black Friday, the largest retail shopping day of the year.

This story has been updated to include comments from Walmart spokesman Dan Fogleman.