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.

Employers can still hold meetings to express their views on unionization, but must now give workers advance notice and make clear they are voluntary.

The National Labor Relations Board ruled yesterday that "captive audience" meetings are illegal. Employers may not require workers to attend presentations that explain to them the company's position on whether they should not join or form unions.