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.

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.

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

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.