Elon Musk's social media company, X (formerly known as Twitter), broke the law when it fired one of its employees for "exercising their right to protected concerted activity" following X's return-to-office mandate, according to a complaint from the National Labor Relations Board.

The complaint alleges that a high-ranking X official told workers, "If you can physically make it to an office and you don't show up, resignation accepted." One employee reached out to coworkers on Slack and encouraged them not to resign but instead wait to get fired. The employee was illegally terminated for their actions, according to the complaint.

The NLRB protects workers' legal right to engage in "concerted activity," which is when two or more coworkers band together to address workplace issues.

A hearing about the complaint is scheduled for January 30 in San Francisco. The NLRB's general counsel intends to prosecute the charge if X and the former employee don't settle first, according to a statement from Kayla Blado, a spokesperson for the NLRB.

X did not respond to CNN's request for comment.

While Musk has said he has cut about 80% of X's staff since taking over the social media company last year, this is the first complaint issued against X by the NLRB.

However, this is not Musk's first run-in with the labor agency.

The NLRB has repeatedly cited Tesla and Musk for illegal or improper anti-union activities, such as interrogating or discriminating against employees who support unionizing. Workers have attempted to organize at Tesla at least three different times.

Musk has also been vocal about his opposition to labor organizing, and the NLRB once directed Musk to delete a 2018 tweet saying Tesla employees would lose their stock options if they formed a union.