Starbucks workers in three more major cities have joined a growing strike that is now in its third of five planned days of escalation ahead of Christmas.

The strike expands to Boston, Massachusetts; Dallas, Texas; and Portland, Oregon, as of Monday. That follows the shutdown of nearly 50 stores nationwide on Sunday, according to a statement from the company’s union, Workers United.

“Nobody wants to strike. It’s a last resort, but Starbucks has broken its promise to thousands of baristas and left us with no choice,” said Fatemeh Alhadjaboodi, a barista and bargaining delegate from Texas.

“In a year when Starbucks invested so many millions in top executive talent, it has failed to present the baristas who make its company run with a viable economic proposal and resolve the pending unfair labor practices,” she said.

The baristas’ union had been in negotiations with Starbucks since February, but claimed that Starbucks “refused to bargain and engaged in bad faith bargaining,” leading to the announcement of a strike last Thursday.

Workers United, which has unionized more than 525 Starbucks locations across the United States, said in a press release Thursday that unfair labor practices and stalled negotiations with the company are the catalyst behind the holiday season strike.

The union said that five days of escalating strikes would continue until Dec. 24 during what it called the company’s busiest days of the year.

“The holiday season should be magical at Starbucks, but for too many of us, there’s a darker side to the peppermint mochas and gingerbread lattes,” said Arloa Fluhr, a bargaining delegate from Illinois who has worked off and on at Starbucks for 18 years, as part of a statement for Workers United.

She said the store’s routine slashing of employee hours meant that she was at risk of being unable to pay her bills — and of losing access to health care, including her daughter’s insulin.

“These walkouts, over unresolved unfair labor practice charges and the company’s failure to offer a serious economic package, are just the beginning,” Fluhr wrote in an op-ed that ran in Fast Company last week.

Workers United and Starbucks had announced in February that they would work on a “foundational framework” to reach a collective bargaining agreement for stores, something the union says has not come to fruition.

In a statement on Thursday following the strike announcement, Starbucks said Workers United delegates had “prematurely ended” its bargaining session earlier in the week.

Workers United, however, claims that the coffee giant has failed to deliver a “serious economic proposal.”

The newest expansion follows a kickoff last Friday in Chicago, Illinois; Los Angeles, California; and Seattle, Washington — which is the home of the brand’s world headquarters.

Other participating locations include Columbus, Ohio; Denver, Colorado; Pittsburgh, Philadelphia; and St. Louis, Missouri.

“This is just the beginning,” Alhadjaboodi said. “We will do whatever it takes to get the company to honor the commitment it made to us in February.”

ABC News’ Zunaira Zaki contributed to this report.