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.