Tesla was required by the National Labor Relations Board (NLRB) earlier this year to post signs at its Fremont, California, factory affirming the right of its employees to engage in union-related activities.

The Guardian first reported the existence of the signs.

An NLRB representative told Business Insider that Tesla was charged with discouraging Fremont employees from wearing union-related stickers and apparel, even though those items did not violate company policies.

The prospect of unionization has been a point of tension at Tesla, as it is the only American automaker whose manufacturing employees are not represented by the United Auto Workers.

Manufacturing workers at foreign automakers with plants in the US — like Toyota, Honda, Nissan, BMW, and Mercedes-Benz — are also not represented by a union.

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Tesla was required by the National Labor Relations Board (NLRB) earlier this year to post signs at its Fremont, California, factory affirming the right of its employees to engage in union-related activities. The Guardian first reported the existence of the signs.

The signs said the following, according to a photo obtained by Business Insider:

"Notice to Employees Posted pursuant to a settlement agreement approved by a regional director of the National Labor Relations Board An agency of the United States government Case: 32-CA-231690 Federal law gives you the right to: Form, join, or assist a union;

Choose a representative to bargain with us on your behalf;

Act together with other employees for your benefit and protection;

Choose not to engage in any of these protected activities. We will not interfere with restrain or coerce you in the exercise of the above rights. We will not tell you that you cannot wear union stickers, order you to remove union stickers from your clothing, or otherwise prohibit you from wearing union stickers at our facility. We will not in any like or related manner interfere with your rights under Section 7 of the Act."

The sign was dated March 22 and said Tesla was required to display it for 60 days.

Tesla did not immediately respond to a request for comment.

Read more: Tesla's Model 3 just earned another big safety award

Case 32-CA-231690 was filed in November 2018 and closed in June following an informal settlement, according to the NLRB's website. An NLRB representative told Business Insider that Tesla was charged with discouraging Fremont employees from wearing union-related stickers and apparel, even though those items did not violate company policies.

The United Auto Workers (UAW) and American Federation of Labor and Congress of Industrial Organizations (AFL-CIO), which are listed as charging parties on the NLRB's website, did not immediately respond to Business Insider's requests for comment, nor did two attorneys associated with the case.

Tesla has a fraught history with unionization efforts

The prospect of unionization has been a point of tension at Tesla, as it is the only American automaker whose manufacturing employees not represented by the UAW. Manufacturing workers at foreign automakers with plants in the US — like Toyota, Honda, Nissan, BMW, and Mercedes-Benz — are also not represented by a union.

A February 2017 Medium post from a Tesla employee describing challenging work conditions at the Fremont factory led to a unionization effort from some employees. The NLRB, UAW, and Tesla employees have alleged that the electric-car maker has prevented them from engaging in union-related activities or retaliated against those who have demonstrated support for a union, claims the company has denied.

See also: Apply here to attend IGNITION: Transportation, an event focused on the future of transportation, in San Francisco on October 22.

Tesla CEO Elon Musk has argued that Fremont employees would not receive enough benefits from union representation to warrant paying union dues, though he attracted controversy last year with a tweet the NLRB interpreted as an attempt to discourage organizing activity.

The UAW itself has dealt with a major controversy in recent years, as it has been the subject of a federal investigation into alleged embezzlement of union funds by some members of its senior leadership.

Are you a current or former Tesla employee? Do you have an opinion about what it's like to work there? Contact this reporter at mmatousek@businessinsider.com.