"Boeing previously took away a pension from our members, and now the company is unwilling to adequately compensate our members' 401(k) plan," the International Association of Machinists and Aerospace Workers union said in a statement. It added that union workers will not accept a contract that is not fair and equitable. "We will not allow this company to put our members’ hard-earned retirements in jeopardy," the union added.

Workers at the three plants in Missouri and Illinois build the F-15, F-18, T-7A trainer, and the MQ-25 unmanned refueler.

Boeing said Sunday it "is disappointed with Sunday’s vote to reject a strong, highly competitive offer. We are activating our contingency plan to support continuity of operations in the event of a strike."

Boeing had offered an additional $3,000 ratificationbonus if workers had approved the three-year contract by Sunday rather than an earlier proposal to make additional $1,000 annual contributions to workers 401(k) plans.

Boeing's 401(k) offer included a company match of workers contributions up to 10% of workers' salaries along with an automatic contribution of 2% for 2023 and 2024.

The current contract is set to expire on Monday, July 25.