The United Auto Workers on Wednesday sharply criticized and vowed to appeal a U.S. agency's decision to let anti-UAW Volkswagen workers defend the results of an election that the union lost last month at a Tennessee VW plant.

The UAW said it would appeal the National Labor Relations Board (NLRB) decision, which it called an outrage.

The union said the NLRB had deviated from its own precedent by giving two groups of anti-UAW workers a formal role as legal parties to the election dispute.

The UAW has asked the NLRB to scrap the results of the election, which the union lost by a 712-626 vote, arguing that anti-union statements by politicians and outside groups compromised voting at the Chattanooga facility.

The anti-UAW workers are supported by the National Right to Work Legal Defense Foundation and Southern Momentum, two of the groups that campaigned against the UAW in the days leading up to the Volkswagen workers' vote.

The UAW said the two groups are "masquerading as legitimate worker representatives", but are actually funded by powerful business interests. They worked in tandem with Tennessee Republican politicians, such as Senator Bob Corker and Governor Bill Haslam, to disseminate anti-union messages, the UAW said.

"It is an outrage that their allies, who refused to reveal their funding sources and who openly republished the illicit threats in the media and among the Volkswagen workforce, will now be allowed to participate in the NLRB hearing," the UAW said in a statement.

The fight over the Chattanooga union election has landed at the feet of the NLRB, a federal agency that supervises union elections and polices unfair labor practices in the private sector.

The NLRB's regional office in Atlanta is handling the UAW's challenge to the election result. That office decided on Monday to grant the two groups of anti-union workers party status in the case. The regional office's decisions can be appealed to the full five-member NLRB board in Washington, D.C.

The UAW said it would ask the board to review the Atlanta office's decision on giving the anti-UAW workers party status.

The Atlanta regional office will also investigate the UAW's claims that outside parties unduly influenced the election outcome at the Chattanooga facility. That process will likely include a hearing, but it has not yet been scheduled.

(Editing by Kevin Drawbaugh and Stephen Powell)