DETROIT (Reuters) - The U.S. National Labor Relations Board has filed an unfair labor practices complaint against Volkswagen AG VOWG_p.DE for hiking health insurance premiums and changing working hours of a group of skilled workers who voted for union representation in 2015.

A general view of the Volkswagen plant in Chattanooga,Tennessee February 14, 2014. REUTERS/Christopher Aluka Berry

The complaint is part of a lengthy battle over the NLRB’s recognition of the vote by roughly 160 skilled workers at VW’s Chattanooga plant in Tennessee to be represented by the United Auto Workers union.

The German automaker has argued against allowing a small group within the plant to have union representation, maintaining that all 1,500 hourly workers should be treated as one unit.

The union sought to represent only a portion of the plant’s workers after it narrowly lost a February 2014 election to represent all hourly workers at the plant.

The NLRB complaint states that issues like working hours and conditions of employment are “mandatory subjects for the purposes of collective bargaining.”

In an emailed statement, VW spokesman Scott Wilson said, “We fundamentally disagree with the decision to separate” the skilled workers from the rest and “will continue our effort to allow everyone to vote as one group on the matter of union representation.”

In an affidavit provide by the UAW, a worker at the plant said that the company has “refused to bargain with the Union over” the changes.

The UAW has never been able to win an organizing vote at a foreign-owned auto assembly plant in the U.S. south. A victory, even for only a portion of workers at the VW plant, would give the union a stronger foothold in the south, where most foreign-owned auto plants are located.

Steven Bernstein, a Tampa-based labor attorney at Fisher Phillips LLP, said the NLRB complaint will likely be followed by others in a legal process that could take three to four years.

“VW is rolling the dice and betting they will eventually get relief through the courts,” he said.

Bernstein said that if VW loses, it could be forced to compensate the skilled workers in Chattanooga. But the consequences of allowing a small group to unionize could be much broader as it would affect other companies across the U.S. south.

“Both sides have a lot to lose in this case,” he said.