A National Labor Relations Board official says it is nearing the end of a probe into a labor contract dispute in which contractors claim union groups are trying to drive up wages on the Volkswagen project.

If the accusations by contractors are rejected, VW could face millions of dollars in added labor costs on its assembly plant, according to the employers group.

“I think we’ll be done by the end of the year,” said Claude Harrell, assistant to the regional director for the NLRB’s Atlanta office.

The investigation stems from a charge by the Associated Mechanical Contractors of Chattanooga, which claims that actions taken by some union groups could boost wage rates while constructing the plant and potentially cost VW money and the area jobs.

It charges that the United Association of Journeymen and Apprentices of the Plumbers and Pipe Fitters Industry in Washington, D.C., wants to impose what would be a $6 per hour increase in prevailing wage rates.

According to a document, the Plumbers and Steamfitters Local Union No. 43 in Chattanooga has a collective bargaining agreement with the mechanical contractors association that remains in effect until June 30.

The Local, through the Mid-South Multi-State Pipe Trades District Council No. 21, says employees are entitled to the higher wage rates than those specified in the agreement.

“Local 43, through District Council 21, has failed to bargain over any change to the existing agreement and has, in effect, repudiated that agreement,” the contractors said.

Judy Gober, the association’s director, said the national union sent word to the local union that the higher rate should be used on the VW plant.

“It will cost VW millions more dollars if they do get to use that rate,” she said. “We’re asking the board that our contract is the one that should be used on that project.”

Officials for the United Association of Journeymen and Local 43 could not be reached for comment Wednesday.

Mr. Harrell said that after the investigation ends it will review the facts and determine if there has been “a probable use violation.”

“If they can’t settle, then we’ll issue a complaint,” he said.

VW is building a $1 billion plant in Chattanooga that is to assemble 150,000 vehicles a year and employ about 2,000 people.