BATH — A Bath Iron Works labor union has vowed to strike if contract negotiations with the shipyard are not successful by Monday, Sept. 25.

The United Automobile Workers’ Local 3999 Bath Marine Draftsmen’s Association decided last weekend to extend its contract by a week so talks could continue. Following several weeks of negotiations, union members rejected a final offer from BIW Sept. 16, and “voted overwhelmingly” to strike if no agreement is reached, according to a Tuesday news release from the union.

“While we continue to engage with the company to negotiate a better proposal for our members and their families, we are also making sure we are as prepared as possible for a strike,” Local 3999 President Kirk Douglass said in the statement.

Other than stating that BIW was continuing communications this week, shipyard spokeswoman Lisa Read declined Monday to comment.

A stumbling block in the talks is BIW’s proposal to cut a flexible work schedule benefit that for the past 15 years has allowed Local 3999 members to continue working while caring for a family.

“Originally developed through a joint union and company effort, the flexible work schedule benefit has been recognized for its proven, positive impact by reducing lost time and providing the opportunity for members to work 40 hours of production each week,” the union release stated.

UAW International expressed its support for Local 3999’s fight for the benefit.

“Flexibility in the workplace has allowed members to balance the needs of their families, including children, aging parents and personal medical and health issues, while designing some of the world’s finest Navy ships,” UAW Region 9A Director Julie Kushner said in the news release.

The UAW represents 760 members at BIW, making it the second-largest unit following production workers in the International Association of Machinists, Brian Rothenberg, senior communications adviser with UAW, said Tuesday.

“The IAM has two union locals and there is an independent union over security as well,” he added.

BIW has about 6,000 employees.

In this Tuesday, Jan. 3 photo, a shipyard worker walks to his car at the end of the workday at Bath Iron Works.