WASHINGTON—The National Labor Relations Board has ruled that employers cannot prevent workers from wearing union apparel or insignias on the job, absent special circumstances that justify such a restriction.

“Wearing union insignia, whether a button or a T-shirt, is a critical form of protected communication,” says NLRB Chairman Lauren McFerran. “For many decades, employees have used insignia to advocate for their workplace interests—from supporting organizing campaigns, to protesting unfair conditions in the workplace—and the law has always protected them.

“With today’s decision, the board reaffirms that any attempt to restrict the wearing of union clothing or insignia is presumptively unlawful and, consistent with Supreme Court precedent, an employer has a heightened burden to justify attempts to limit this important right.”

The ruling came in a suit brought against Tesla Inc., which had instituted a dress code that required employees to wear a plain black T-shirt or one imprinted with the Tesla logo, thus prohibiting employees from substituting a shirt bearing union insignia. Tesla argued that its dress code was meant to prevent workers’ clothing from “causing mutilations” to cars and to help managers “easily determine that employees are in their assigned work areas.”

Former Tesla employees had testified before the NLRB that management at Tesla made them remove T-shirts with United Auto Workers union messages and logos on them, even though they posed no risk of damaging cars or car seats.