A timeline is set for a new unionization vote at an Amazon warehouse.

Amazon workers in Alabama will have another shot at voting to form a union this winter, as the National Labor Relations Board scheduled a mail-in election to start Feb. 4.

The agency told the workers on Tuesday that they would have until March 25 to submit their votes. The agency will count the ballots on March 28.

Workers at an Amazon warehouse near Birmingham, known as BHM1, had voted against forming an union last spring after a highly contentious and public campaign. The labor agency threw out the results after finding Amazon improperly interfered with the election, opening the door for a new vote.

The agency told the workers on Tuesday that in the original vote, Amazon “interfered with the employees’ exercise of a free and reasoned choice” by having a mailbox installed at the warehouse, “creating the appearance of irregularity,” and polling workers about their opinion. Amazon, which has said the mailbox was intended to make voting easier, has not appealed the decision to rerun the election.

The prominent organizing effort has been run by the Retail, Wholesale and Department Store Union. It has drawn national attention from politicians and other figures as it aims to form the first union at one of the nation's largest employers.

Barbara Agrait, an Amazon spokeswoman, said in a statement that Amazon’s “employees have always had the choice of whether or not to join a union, and they overwhelmingly chose not to join,” adding that the company looked forward to the workers “having their voices heard again.”

The union said in a statement that it had asked the agency to provide “a number of remedies that could have made the process fairer to workers” in the new election, but that the agency did not approve their requests. “We are deeply concerned that the decision fails to adequately prevent Amazon from continuing its objectionable behavior in a new election,” the union said.

The company, which has vowed to become a “Earth’s best employer,” is facing labor pressure on multiple fronts. It has said it would spend $4 billion in the holiday quarter alone to deal with labor shortages. Workers in Staten Island are also trying to unionize, and last month Amazon and the labor board signed an unusually broad nationwide settlement giving workers more power to organize.