In a warning to employers, the National Labor Relations Board has “unliked” certain social media policies that restrict an employee’s right to speak critically of the employer online, unless the policies were set in collective bargaining.

In a recently released advice memo [PDF], the agency says social media policies are work rules that provide a basis for discipline, and as such must be the subject of bargaining unless a union waives that right.

The memo was written by the NLRB’s Office of General Counsel and was obtained through a Freedom of Information Act request by Law360. It involves the Giant Food company and its three unions.

“The memo also reemphasized prior GC social media pronouncements to find that certain provisions of Giant Food's policy—commonly found within employee handbooks—infringed on its employees' National Labor Relations Act (NLRA) rights,” according to Joel Barras, labor and employment counsel at Reed Smith in Philadelphia, writing on the firm’s Employment Law Watch blog.

The employer’s policy, according to the memo, contained at least three guidelines that prohibited releasing confidential company information, using the company logo or other visual elements, or discrediting the company’s products. Another guideline told employees to speak up if someone was violating the guidelines or “misusing” a company website.

The Office of General Counsel found some of this language too vague and overreaching, because an employee could read it to prohibit sharing information on wages or work conditions.

The NLRA protects the rights of employees to speak and to act together to address work conditions. Since 2010, the NLRB has extended this protection to work-related conversations on social media like Facebook and Twitter, and the Office of General Counsel has issued complaints against employers who issue policies that overly restrict employees’ conversations on social media.

The Giant Food memo concludes that “portions of the employer’s social media policy violate [the NLRA] because they would reasonably be construed to chill” protected employee activity under the Act.

Meanwhile, the U.S. Senate is expected to vote next week to confirm President Barack Obama's two nominees to the NLRB. Nancy Schiffer, former associate general counsel to the AFL-CIO, and Kent Hirozawa, chief counsel to the NLRB’s chairman, were nominated last week as part of a compromise with Republicans on the confirmation process.

The two testified at a Senate hearing on Tuesday and vowed to be impartial members of the board.

Sue Reisinger is a reporter for Corporate Counsel, a Legal affiliate based in New York.