SEATTLE — Boeing Co. aircraft assembly workers have voted to strike for an unprecedented second time in three years. But their contract has been extended 48 hours at the request of Washington Gov. Chris Gregoire and a federal mediator.

The vote late Wednesday was 87 percent in favor of a strike as unanimously recommended by machinists union negotiators. Under union rules a strike requires at least two-thirds support from those voting.

In separate balloting, union members also voted 80 percent to reject Boeing's third and final three-year contract offer, which included pay raises averaging 11 percent.

The walkout was to begin at 12:01 a.m. PDT Thursday, after the expiration of a contract covering more than 27,000 workers. Now that contract has a short extension.

The union's vote was to try to force the planemaker back to the table with a proposal that provides more job security.

Eighty percent of the voters opposed the contract, the International Association of Machinists and Aerospace Workers said Wednesday night. The union represents about 27,000 employees in Washington state, Oregon and Kansas who make parts and assemble planes for Boeing, the world's second-largest commercial aircraft maker. Boeing has about 740 employees in Utah.

Chicago-based Boeing offered an 11 percent raise over three years, plus bonuses that included $2,500 if the deal had been ratified today by at least half the voters. The company didn't go along with changes the union sought concerning the use of outside contractors for work the machinists have traditionally done.