The complaint from the National Labor Relations Board claims that Amazon is a so-called "joint employer" of drivers employed by the contractor, Battle Tested Strategies (BTS), and used a series of illegal tactics to discourage union activities at a facility in Palmdale, California.

BTS drivers voted to join the International Brotherhood of Teamsters union last year, becoming the first Amazon delivery contractors to unionize.

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.

The board in August had said that it had found merit to the union's claims that Amazon exerts control over BTS drivers and should be considered their employer under federal labor law. The NLRB at the time said it would issue a complaint unless Amazon settled the case.

Last month, the board said it planned to issue a second complaint involving a different group of Amazon drivers.

Amazon did not immediately respond to a request for comment. The company has said in the past that it does not have enough control over drivers' working conditions to be considered their joint employer.

Joint employment has been one of the most contentious U.S. labor issues over the last decade, and the NLRB's standard for determining when companies qualify as joint employers has shifted numerous times since the Obama administration. Business groups favor a test that requires direct and immediate control over workers, while unions and Democrats back a standard that covers indirect forms of control.

The case will be heard by an administrative judge in Los Angeles, who is scheduled to hold an initial hearing next March. The judge's decision can be reviewed by the five-member NLRB, whose rulings can be appealed to federal court.

A ruling that Amazon is a joint employer under federal labor law could be applied in cases involving other Amazon contractors and force the company to bargain with drivers' unions.

The board, meanwhile, is facing claims by a growing number of companies, including Amazon, that its structure and in-house enforcement proceedings violate the U.S. Constitution.

Amazon has filed a lawsuit against the board seeking to block it from deciding whether the company must bargain with a union representing workers at a New York City warehouse. A federal appeals court on Monday temporarily blocked the NLRB from ruling while it reviews Amazon's claims.