Starbucks on Monday asked the National Labor Relations Board to temporarily suspend all union elections at its U.S. stores, citing allegations from a board employee that regional NLRB officials improperly coordinated with union organizers.

In a letter to the board chairman, Starbucks said the unnamed career NLRB employee informed the company about the activity, which happened in the board's St. Louis office in the spring while it was overseeing a union election at a Starbucks store in Overland Park, Kansas.

The store is one of 314 U.S. Starbucks locations where workers have petitioned the NLRB to hold union elections since late last year. More than 220 of those Starbucks stores have voted to unionize. The company opposes the unionization effort.

The Seattle coffee giant alleges that St. Louis labor board officials made special arrangements for pro-union workers to vote in person at its office when they did not receive mail-in ballots, even though Starbucks and the union had agreed that store elections would be handled by mail-in ballot.

In its letter, Starbucks referred to memos the regional office sent confirming that workers were allowed to come to the office and vote in person after the union told the regional office that some workers had not received ballots in the mail. The memos, citing "board protocol," said the workers voted alone in an empty office, according to Starbucks.

"Because observers were not present, no one can be sure who appeared to vote, whether NLRB personnel had inappropriate communications with the voters, told them how to vote, showed them how to vote or engaged in other undisclosed conduct," Starbucks wrote in its letter.

Starbucks said regional board officials also disclosed confidential information to the union, including which workers' ballots had arrived in the mail to be counted.

Starbucks Workers United, the group seeking to unionize U.S. Starbucks stores, accused the company of trying to "distract attention away from their unprecedented anti-union campaign, including firing over 75 union leaders across the country, while simultaneously trying to halt all union elections."

"Workers have spoken loud and clear by winning 82 percent of union elections," the group said in a statement. "Ultimately, this is Starbucks' latest attempt to manipulate the legal process for their own means and prevent workers from exercising their fundamental right to organize."

A spokesperson for the NLRB said Monday that the agency doesn't comment on open cases.

Press secretary Kayla Blado said the NLRB will "carefully and objectively" consider any challenges that Starbucks raises through "established channels." Starbucks can also seek expedited review in the case, Blado said.

Workers at the Overland Park store petitioned the NLRB to hold a vote in February. In April, workers voted 6-1 to unionize, but seven additional ballots were the subject of challenges from Starbucks or the union. A hearing on those challenges is scheduled for Tuesday; Starbucks has asked for that hearing to be delayed.

Starbucks said there is evidence of misconduct in other regions as well. The company wants the NLRB to thoroughly investigate other Starbucks union elections and make public a report on its findings. The company said the board should also implement safeguards to prevent regional officials from coordinating with one party or another.

Starbucks also asked the NLRB to issue an order requiring all pending and future elections to be conducted in person with observers from both sides.

"If the NLRB does not respond by investigating and remedying these types of actions, we do not see how the board can represent itself as a neutral agency," the company said in the letter.

Starbucks has long opposed unionization, dating back to CEO Howard Schultz's acquisition of the company in the late 1980s. The current unionization effort has been riddled with accusations and lawsuits on both sides.

Starbucks Workers United has filed 284 unfair labor practice charges with the NLRB against Starbucks or one of its operators, according to the labor board. Starbucks has filed two charges against Workers United.

Earlier this month, the labor board dismissed one of the charges filed by Starbucks, saying the company failed to prove that pro-union workers blocked store entrances or intimidated customers during a spring rally.

In June, the NLRB asked a federal court in western New York to order Starbucks to stop interfering with unionization efforts at its U.S. stores. It also asked the court to order Starbucks to reinstate seven Buffalo employees it says were unlawfully fired for trying to form a union. That case is pending.

But the NLRB's actions against Starbucks haven't always been successful. In June, a federal judge in Phoenix ruled that Starbucks didn't have to rehire three workers who claimed that the company had retaliated against them for organizing a union.

Starbucks isn't the only large company facing a unionization effort that has attacked the voting process.

Amazon has also levied accusations of improper conduct against the NLRB's regional office in Brooklyn in its attempt to re-do a historic labor win at a warehouse on Staten Island, New York. Among other allegations, Amazon said the agency tainted the voting process by seeking reinstatement of a fired Amazon worker in the weeks leading up to the March election.

Attorneys representing the e-commerce juggernaut argued their case in an agency hearing during the summer. Attorneys for the agency have pushed back. A regional director for an NLRB office in Phoenix is expected to issue a ruling on that case in the coming weeks.

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Associated Press Business Writer Haleluya Hadero in New York contributed to this report.