Starbucks has fired a number of workers leading efforts to organise a union in Tennessee, one of more than 50 such drives it is facing across the US.
The coffee chain said the staff had knowingly violated company rules, including by using a store after-hours.
Labour organisers said the firm was retaliating in a bid to slow the momentum of their efforts.
Buffalo, New York recently became home to the first unionised Starbucks-owned stores in the US since the 1980s.
Since then, dozens more locations of the chain have also filed to hold votes about joining a union, which would give workers the ability to negotiate as a group with the firm over pay and conditions.
Starbucks spokesman Reggie Borges rejected claims of retaliation, noting that the firm has not stopped staff involved in the ongoing union efforts from speaking out.
He said the firm did an internal investigation, including interviews with those involved, which confirmed staff had been aware of the rules against accessing stores after hours without permission when they used the cafe for a January television interview about the union efforts.
He did not provide figures for how many people have lost their jobs for breaking such policies but said this case was clear cut.
Starbucks Workers United, which has been helping to spearhead efforts, said that it planned to file charges over the seven firings at the store in Memphis with the National Labor Relations Board, which enforces US labour laws.
It said the coffee chain was selectively enforcing policies as a "subterfuge" to fire union leaders. Of the seven people who lost their jobs, most had impeccable work records, the group said.
"It definitely feels like anti-union tactics and union-busting tactics because these are infractions that would have never been written up," said former barista LaKota McGlawn, who had worked for Starbucks since December 2020, and was promoted to a shift supervisor last year, earning $15.25 an hour.
'Determined to fight'
The 22-year-old, one of the members of the local organising committee, said she never received training regarding some of the rules in question and the firing took her by surprise, despite a meeting with supervisors following the TV interview.
"This was honestly one of the last things I was expecting today," she told the BBC. "We're determined to fight this."
Starbucks has consistently opposed staff efforts to join a union, which it says would make the company less able to respond quickly to worker concerns.
Last year, the National Labor Relations Board found the company had retaliated illegally against two baristas involved in labour organising in Philadelphia, a decision the firm has appealed.
Executives recently told investors that efforts to keep and train staff were weighing on its bottom line. It has more than 8,000 company-owned cafes across the US and at one time was known for standing out among retail employers for the benefits it offered.