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.

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 NLRB said that Amazon managers solicited grievances and made implied promises to remedy them if workers rejected the union during mandatory meetings at the Staten Island wareh