By Daniel Wiessner

Activision Blizzard Inc violated U.S. labor law by illegally surveilling employees during a walkout and threatening to shut down internal chat channels as a union sought to organize its workers, a federal labor agency said on Friday.

A National Labor Relations Board spokeswoman said that unless Activision settles, the agency will issue a complaint against the company involving employees of its subsidiary Blizzard Entertainment Inc based in California and three other states.

The Communication Workers of America union (CWA) has accused the "Call of Duty" maker of a series of illegal labor practices at the union has sought to organize video game testers and other employees at the company and its subsidiaries.

Blizzard employees around the country staged a walkout last year to protest what they said was a lack of gender equality at the company.

Kayla Blado, a spokeswoman for the labor board, said on Friday that a regional agency official had found merit to the CWA's claim that Activision used security staff to keep tabs on workers during the walkout.

A claim that the company also broke the law by threatening to close internal Slack channels where employees frequently discussed working conditions was also found to have merit, Blado said.

An Activision spokesperson in a statement defended the company's ability to prevent "toxic workplace behavior."

"CWA wants us to accept their... false claims, but we strongly believe employees shouldn't have to be subjected to insults and put downs for their hard work – especially on company communication platforms," the spokesperson said.

The union did not immediately respond to a request for comment.

Activision is already facing a separate NLRB complaint issued last year claiming the company used a policy limiting what workers can post on social media to bar them from discussing working conditions. Activision has said its social media policy is lawful and does not bar employees from exercising their rights under U.S. labor law.

Small groups of workers at Activision subsidiaries in New York and Wisconsin have voted to join the CWA in recent months, and employees in Boston are seeking to have an election. Activision has said it is considering its options in those cases.

Xbox maker Microsoft Corp last year agreed to buy Activision for $69 billion, a deal that has faced antitrust scrutiny from U.S. and European regulators.

(Reporting by Daniel Wiessner in Albany, New York; Editing by Alexia Garamfalvi and Rosalba O'Brien)