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

The decision passed 3-1, with the NLRB General Counsel, Jennifer Abruzzo, arguing the meetings violated worker’s rights, as they were forced to attend and be subjected to the company’s anti-union messaging.