(Reuters) -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 complaint from the National Labor Relations Board (NLRB) 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 a complaint 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 had said in August 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.

The board said last month it planned to issue a second complaint involving a different group of Amazon drivers.

In a statement, Amazon spokesperson Eileen Hards said the NLRB did not include many of the Teamsters' claims in its complaint, showing that the union was "misrepresenting the facts."

"As we've said all along, there is no merit to any of their claims. We look forward to showing that as the legal process continues and expect the few remaining allegations will be dismissed as well," Hards said.

Amazon has said in the past that it does not have enough control over drivers' working conditions to be considered their joint employer.

Teamsters President Sean O'Brien said in a statement that Amazon is trying to reap the benefits of drivers' labor without taking responsibility for their well being.

"This decision brings us one step closer to getting Amazon workers the pay, working conditions, and contracts they deserve," O'Brien said.

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.

(Reporting by Daniel Wiessner in Albany, New York; Editing by Alexandra Hudson, Mark Porter and Muralikumar Anantharaman)