Starbucks workers at a Buffalo store form union in a contentious vote.

The National Labor Relations Board announced on Thursday that workers at a Starbucks store in Buffalo voted to form a union.

The labor model at the coffee retailer is challenged by the result. Before the vote, no Starbucks stores in the US were unionized. The votes will be counted at two other stores.

Problems that have plagued the company for years but which appeared to get worse during the swine flu, were cited by workers when they filed for union elections at the stores in late August.

The election took place through mail ballots. In November, workers at three more Buffalo-area stores filed the paperwork needed to hold union elections, but it was unclear when votes would take place.

Starbucks responded to the campaign quickly. Managers and executives from out of town came to Buffalo to question employees about operational challenges and help with menial tasks.

A district manager from Arizona told her co-workers that the company had asked her to go to Buffalo to save it from unionization, in a video that was viewed by The New York Times.

Several workers who support the union said they found the presence of these officials intimidating. They complained that Starbucks had temporarily closed certain stores in the area, which they found disruptive, and that Starbucks had added staff in at least one of the stores that held elections. The workers said this made them less likely to support unionization.

According to former National Labor Relations Board officials, these actions by the company could be seen as an attempt to undermine the laboratory conditions that are supposed to prevail during union elections. The regional director of the labor board overturned a union election at an Amazon warehouse in Alabama.

The former labor board officials said that if the additional workers did not appear to serve a legitimate business purpose and were likely to oppose the union, it could be considered illegal to pack a store.

Starbucks has said that it dispatched out-of-town officials and temporarily closed stores to help solve staffing and training problems and to make them more efficient. The company said that it added staff to deal with an increase in the number of workers calling in sick and that it has taken such steps across the country since the spring, when coronavirus infection rates dropped and stores became busy.

Rossann Williams, the North America president, said in an interview on Wednesday from Buffalo that she did not feel that the run-up to the vote had been particularly contentious and that she had spent a lot of her time there listening to employees.

The company said that it did not believe that any of its actions would prompt the labor board to throw out the results.

The labor board ordered Starbucks to hold separate elections for workers at its 20 stores in the Buffalo area, but Starbucks argued that workers should vote together in a single election. The company said that allowing individual stores to decide whether to unionize is problematic because employees can work multiple locations and the stores are largely managed as a group. A single election favors the employer.

Starbucks filed an appeal to the National Labor Relations Board in Washington to block the elections. The board denied the request for a review.

Starbucks has faced union campaigns in the past, including one in New York City in 2000 and one in Philadelphia in 2019. A decision is still pending after the company appealed the ruling.

Starbucks workers are unionized at stores owned by other companies that operate them under licensing agreements. Workers at a company-owned store in Canada have unionized.