(Updated Sept. 13, 9:40 a.m. ET with reaction from analysts)

More than 30,000 aerospace workers at Boeing will walk off the job Friday after a large majority of them rejected a tentative contract.

The International Association of Machinists and Aerospace Workers announced the result on its website late Thursday night after vote tabulation was completed. More than 94% of union members voted and 96% of those voted against the deal.

The union said its negotiating team will “regroup and begin planning the next steps on securing an agreement that our membership can approve.”

IAM chief Jon Holden said earlier this week that the tentative deal was the best contract the union has ever negotiated.

Union leaders and Boeing (NYSE: BA) reached a tentative agreement on Sunday, but rank-and-file machinists immediately expressed unhappiness with it. The last contract was ratified 16 years ago. On Monday, workers rallied outside Boeing’s widebody aircraft plant in Everett, Washington.

One of the biggest issues is pay. Boeing is offering a wage increase of 25% over four years, but union members say it doesn’t include their current yearly bonus. Boeing will also put more matching funds into a 401(k) retirement plan but won’t restore a pension plan that machinists used to have and gave up in an earlier negotiation. The collective bargaining agreement would also change mandatory overtime so workers would be required to work overtime and weekends less often, and it puts in a floating vacation holiday. Workers would also be able to progress through different job responsibilities more easily.

Workers also complain that the wage increase isn’t high enough, that the pension wasn’t restored and that Boeing’s promise to build a future aircraft type in the Puget Sound region only extends for the length of the contract. Workers still feel betrayed about previous rounds of negotiations in which Boeing twice threatened to move jobs outside of Washington.

IAM District 751 strikes historically have lasted about 60 days, but the precedent of Spirit AeroSystems’ negotiated contract in June 2023 suggests the strike could be as short as a week, Bank of America analyst Ronald Epstein said in a client note. It is likely Boeing will have to make further concessions and move closer to the IAM’s initial proposal of 40% wage gains, he added.

If a strike goes on for an extended period, the delivery of aircraft and components would stop. That won’t have an immediate effect on airlines but would exacerbate the current backlog of aircraft and further limit the ability of airlines to modernize or expand their fleets to reach new markets.