Talks between shipping giant the United Parcel Service (UPS) and the International Brotherhood of Teamsters fell apart last week, upping the possibility that the union could strike in a massive walkout when their current contract expires at the end of this month.

The union, which represents roughly 340,000 members working for UPS, voted last month to authorize a strike if a deal isn’t reached before July 31. The strike would be one of the largest in U.S. history.