The National Labor Relations Board has ruled it is illegal to force union-backing employees to attend meetings held by their companies that are designed to undermine support and argue against the labor groups.

The ruling, stemming from a complaint against Amazon, bars companies from compelling workers to attend meetings on unionization’s downsides.

Amazon.com made unlawful promises and threats during mandatory meetings to discourage unionizing at a New York City warehouse, a U.S. labor board said on Wednesday while banning employers from holding such meetings moving forward.

After a lengthy consideration, the National Labor Relations Board has ruled that Amazon’s “captive-audience meetings” are a violation of the National Labor Relations Act.

Mandatory “captive audience” meetings in which companies argue against unionization are illegal, the National Labor Relations Board ruled in a case involving Amazon.com Inc., prohibiting one of employers’ most potent weapons against labor organizing campaigns.

The National Labor Relations Board ruled that employers can’t force workers to listen to anti-union speeches, but a Trump board could reverse the decision.

The National Labor Relations Board announced a new ban on mandatory captive-audience meetings aimed at discouraging employees from unionizing.