In August the company behind "Call of Duty" and "World of Warcraft" promised change with a management shakeup following long-running allegations of sexism and discrimination.

The Communications Workers of America (CWA) said Tuesday they had "formally filed ULP (unfair labor practice) charges against the company for worker intimidation and union busting."

In a statement, the union said employees had launched a "Better ABK" campaign following a July complaint, filed in Los Angeles Superior Court, accusing the firm of having fostered a toxic culture of harassment and sexism.

It was part of workers' "strategic organizing effort to unite developers behind a shared set of concerns."

The CWA said Activision Blizzard was "using coercive tactics to attempt to prevent its employees from exercising their rights."

"Management could have responded with humility and a willingness to take necessary steps to address the horrid conditions some ABK workers have faced," said Tom Smith, CWA's national organizing director.

"Instead Activision Blizzard's response to righteous worker activity was surveillance, intimidation, and hiring notorious union busters."

The allegations follow a walk-out earlier this year to protest sexism and harassment.

In August, the firm unveiled a management shakeup and launched what it promised would be a far-reaching review of its workplace practices after the California state lawsuit.

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