AUGUSTA — The largest union at Bath Iron Works has sued the shipyard, alleging the company violated its contract with the workers by seeking to modify job descriptions outside of a formal contract negotiation.

“There are more than 100 changes and additions to people’s jobs and tasks,” said Jay Wadleigh, president of the International Association of Machinists Union Local S6.

While the union might object to the nature of those changes, the lawsuit filed Wednesday in U.S. District Court in Portland takes issue with the company’s demand to resolve the job description changes before a third party, in arbitration.

The lawsuit comes as tensions between the company and union are high and its leaders focus on increasing efficiency and cutting costs, primarily to win a contract for the next batch of U.S. Coast Guard cutters.

“Because they’re trying to cut costs and find ways to get our bid down to the Coast Guard, they want to — in the middle of the contract — change our job descriptions,” Wadleigh said. “It’s not the appropriate time to do that.”

The union’s contract expires in May 2016.

Wadleigh said the matter has already been scheduled for a May 29 arbitration meeting. The union plans to argue then that the changes are not a dispute that can be settled through arbitration and filed the lawsuit in federal court to have another venue for disputing that issue.

In the lawsuit, the union alleges that BIW demanded to take the proposed job function changes to arbitration on April 3, citing a 2001 memorandum of understanding that outlines how changes could be made to the “Classification Task List” governing which tasks certain types of workers can do.

The union contests that the company’s broader effort to change job descriptions and tasks falls outside that agreement and should instead be the subject of negotiations leading into the 2016 contract.

Separately, the union has disputed through arbitration the company’s plan to buy certain components of its ships from other vendors rather than make them at the shipyard. Fred Harris, the shipyard’s president, said in January that BIW is out of line with the rest of the industry in making ship components on-site and that the process adds expense to its shipbuilding.

Wadleigh said the company proposed sourcing 11 components from outside the shipyard rather than making them. Through arbitration, Wadleigh said the union convinced the company that two of those components are actually cheaper to make on-site. The nine others, he said, remain a point of contention for upcoming arbitration meetings.

A spokesman for Bath Iron Works was not immediately available for comment on the lawsuit.