Workers who are fighting for a union contract at REI have authorized a boycott of the outdoor retailer during its biggest sale of the year.

The union said Wednesday that an “overwhelming majority” of workers from 11 organized stores voted in favor of the move this week. If the boycott proceeds, the union would be asking customers not to shop REI’s anniversary sale, scheduled in May.

“The company is now unilaterally implementing cuts to benefits, starting wages, and raises for REI Union workers, which led union members to take this critical next step,” the union said in a statement.

A final call on the boycott would be made by May 1, the union added, leaving open the possibility that the two sides could make progress in negotiations.

As HuffPost reported last week, the boycott threat is a major escalation in the four-year labor battle at REI. Workers at nearly a dozen of the chain’s 190 stores have joined the United Food and Commercial Workers and its sister union, the Retail, Wholesale and Department Store Union, but they’ve faced strong pushback from leadership despite REI’s progressive reputation.