A high-profile labour organiser in the United States has resigned from Starbucks, saying the company forced her out because of her union leadership.

Jaz Brisack, a barista who helped lead the unionisation of a store in downtown Buffalo, New York, late last year, said on Wednesday that her last day at the company will be September 18.

The vote at Brisack’s store kicked off a movement; since then, at least 238 US Starbucks stores have voted to unionise, according to the National Labor Relations Board (NLRB).

In a letter to her manager, which Brisack shared with The Associated Press news agency, Brisack said Starbucks has refused to accommodate her availability requests for seven months.

Brisack said that has hurt morale at the store, where her co-workers have had to cover for her when she is absent.

“Starbucks has deliberately made my continued employment at the company impossible,” said Brisack, who has worked at the company for nearly two years.

Seattle-based Starbucks said it tried to balance Brisack’s scheduling requests with the store’s staffing needs. The company said Brisack was working about 20 hours per week until May, when she told the store she was only available for 6.5 hours on one day per week. Starbucks said that was not approved because it did not meet the store’s needs.

“We work to treat every partner equally, balancing their scheduling requests with the business and customer needs of the store,” Starbucks spokesman Reggie Borges said.

Brisack said her request was not unusual, and many people work at Starbucks only one or two days per week. Borges said schedules vary by store, but Brisack’s store is already so understaffed that it often has to close early.

Starbucks does not support the unionisation effort. But Borges said no employee is treated differently or disciplined because of their support for unions.

Brisack said at least 10 of her co-workers have been fired by the company during the last year. In June, the NLRB filed a federal court case in New York seeking the reinstatement of seven pro-union workers who were fired from a store in Buffalo.

The NLRB has also charged Starbucks with interfering with workers’ right to organise in Memphis, Tennessee, where the company fired seven workers in February. A federal judge in Memphis recently ordered Starbucks to reinstate those workers while the NLRB case plays out.

But the NLRB lost a similar case in June, when a federal judge in Phoenix denied the agency’s request to force Starbucks to rehire three workers.

Workers United, the union backing the Starbucks drive, said on Wednesday that it has filed an unfair labour practice charge against Starbucks on Brisack’s behalf.

Brisack said she expected the NLRB will order Starbucks to reinstate her. In the meantime, she will remain on the bargaining committee for her store and will continue to work with Workers United to organise other Starbucks stores.