Amazon.com has been accused by a U.S. labor board of illegally refusing to bargain with a union representing drivers employed by a contractor, the agency announced on Wednesday.

The U.S. National Labor Relations Board (NLRB) has accused Amazon of unlawfully refusing to negotiate with a union representing drivers from Battle Tested Strategies (BTS). Allegedly a joint employer,

The NLRB in the complaint, which was issued on Monday, said Amazon broke the law by terminating its contract with BTS after the drivers unionized without first bargaining with the Teamsters