The National Labor Relations Board says Starbucks is violating U.S. labor law by withholding pay hikes and other benefits from stores that have voted to unionize.

The labor board’s Seattle office filed the complaint late Wednesday against Starbucks. The complaint is based on charges filed by Workers United, the union trying to organize Starbucks’ 9,000 company-owned U.S. stores.

The complaint adds to an already lengthy paper trail in the acrimonious relationship between Starbucks — which opposes unionization — and Workers United. More than 220 U.S. Starbucks stores have voted to unionize since late last year.

The complain is one of at least 20 that NLRB regional offices have filed against Starbucks alleging unfair labor practices. Starbucks has also filed complaints against the board and the union. Last week, the company asked the NLRB to halt union elections entirely, saying it has evidence that a regional office improperly coordinated with union officials. A decision in that case is pending.

In the case filed Wednesday, the NLRB said Starbucks violated labor law by offering raises and benefits — including increased training, career development opportunities, expanded tipping and even looser dress code policies — only to non-union stores.

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Starbucks announced the $200 million in added worker pay and benefits in May after a series of meetings with workers around the country. At the time, Starbucks interim CEO Howard Schultz said U.S. labor law requires union stores to negotiate their own contracts with the company.

“We do not have the same freedom to make these improvements at locations that have a union,” Schultz said in a conference call with investors.

Starbucks reiterated that argument in a statement Thursday.

“Wages and benefits are mandatory subjects of the collective bargaining process,” the company said. It rejects the union’s argument that it could offer the wage and benefit enhancements to unionized stores at any time.

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But in its complaint, the NLRB’s regional office said Starbucks’ action violates laws that prohibit interfering with employees’ rights to organize.

The complaint seeks back pay for affected employees. It also would require Schultz to hold an employee meeting and a read a statement explaining workers’ right to organize.

The regional complaint will be heard by an administrative law judge at the NLRB. Once a decision is reached, either side can appeal to the full National Labor Relations Board in Washington.