Jan. 15 (Bloomberg) — Wal-Mart Stores Inc. illegally retaliated against protesting U.S. workers, the National Labor Relations Board said in a complaint a union called one of the largest ever filed against an employer.

The labor board said Wal-Mart, the world's largest retailer, threatened, disciplined and fired employees who took part in strikes and protests at 34 stores in 14 states over the past two years.

"Walmart thinks it can scare us with attacks to keep us from having a real conversation about the poverty wages we're paid," Barbara Collins, a fired Wal-Mart worker from Placerville, California, who is one of the workers named in the complaint, said in a statement. "Now the federal government is confirming what we already know: We have the right to speak out, and Walmart fired me and my coworkers illegally."