Leaders of Boeing Co.'s Machinists union called for a strike Friday after deciding a proposed labor contract the aerospace company called its "best and final" offer wasn't good enough. They urged members to reject the offer in an upcoming vote.

The Chicago-based company hoped the proposal, which provides added pay and incentives to workers over three years, would help it avert a labor standoff. The talks come as Boeing tries to keep up with a backlog of plane orders and avoid more penalties caused by production delays of its next-generation passenger jet.

A Boeing spokesman said the company was "extremely disappointed" by the union's response. Boeing has about 740 employees in Utah.

The sides have been negotiating over a new three-year contract since May 9, and round-the-clock talks began Aug. 21 at a Seattle airport. In 2005, about 18,400 machinists in the Pacific Northwest and Wichita, Kan., struck for four weeks, forcing the company to halt commercial airplane production. The machinists assemble Boeing's commercial planes and some key components.

The proposal, Boeing's third offer, was delivered to the union Thursday. It would have increased pay by 11 percent on average for more than 27,000 union workers in Washington state, Kansas and Oregon, the company said.

The tentative deal also included a $2,500 bonus for workers if the agreement was ratified by Wednesday, when the current contract expires and a union vote is scheduled.

Boeing said it had withdrawn certain contentious proposals, such as plans to cut early retiree medical coverage and create a new defined-contribution retirement program for future employees.

The union held a preliminary vote to authorize a strike in July. On Wednesday, members are scheduled to cast two ballots: one to accept or reject Boeing's latest offer and another on whether to launch a strike. A simple majority is required to reject the contract, and a two-thirds majority is needed to call a strike, which would trigger a work stoppage at 12:01 a.m. Thursday.

Tim Healy, a Boeing spokesman, said: "We're extremely disappointed that the union is recommending that our employees reject what adds up to the best contract in the aerospace industry."

"We hope our employees recognize the value of this offer," he said. "We encourage them to study it, calculate what it means to them and their families and vote in their best interests on Sept. 3. This is our best and final offer, and that's what it means."

Analysts say a strike could cost Boeing about $100 million per day in deferred revenue. In 2005, Boeing was unable to deliver more than two dozen airplanes as scheduled because of the strike that year.