BATH — Bath Iron Works and its largest union have reached a tentative agreement on a four-year contract, both sides said Thursday.

The contract improves pensions and wages and “holds the line” on health care costs for employees, said John Carr, a spokesman for the International Association of Machinists and Aerospace Workers.

“The negotiating committee brought back a tentative agreement that is the leader in the shipbuilding industry,” Carr said.

Additional details on wages and other provisions of the contract proposal were not immediately available.

The union, which represents about 3,200 mechanics, fitters, welders, painters and electricians, will vote on the tentative deal Sunday at the Augusta Civic Center.

“We’re pleased the negotiating teams have achieved a tentative agreement. We believe this is a fair and equitable offer which provides competitive wages and benefits to our employees while placing the company in a good position to compete for future shipbuilding contracts,” said Jim DeMartini, a spokesman for BIW.

The tentative agreement comes at a crucial time for BIW, which employs about 5,400 workers.

The company, which depends on defense contracts from the Navy, is bidding on a long-term contract. The Navy is expected to decide later this year on contracts for nine destroyers. BIW, which is a unit of General Dynamics Corp., is competing against Huntington Ingalls Industries for the DDG-51 destroyers the Navy will buy through fiscal 2017.

The tentative agreement was reached at midnight Wednesday, said the company. The Local S6 negotiating committee is unanimously endorsing the company’s offer to the membership, BIW said.

If union members reject the contract proposal, a strike authorization vote will be held. A strike could begin at 12:01 a.m. May 21 if the proposal fails, Carr said. Bath Iron Works has not had a strike since a 55-day walkout in 2000.