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Boeing is offering its staff a 25% pay rise over four years in a bid to avoid a strike that could potentially shut down its assembly lines as early as Friday.

Union leaders representing more than 30,000 employees have urged the workers to support the proposal, describing it as the best contract they had ever negotiated.

If approved, the agreement would be an important achievement for Boeing's new chief executive, Kelly Ortberg, who faces pressure to fix the company's quality and reputational issues.

Boeing workers in the Seattle and Portland region are set to vote on the deal on Thursday. If the deal is rejected, however, a second ballot will need to be approved by two-thirds of union members for a strike to go ahead.

In a video message to Boeing workers, the aerospace giant's chief operating officer, Stephanie Pope, described the proposal as a "historic offer".

If ratified by members of the International Association of Machinists and Aerospace Workers (IAM) union, it would be the first full labour agreement between the firm and the unions in 16 years.

The current contract between Boeing and the unions was first reached in 2008 following an eight-week strike. The two sides agreed to extend it in 2014 and it is now due to expire later this week.

Although the preliminary deal did not match the union's initial target of a 40% pay rise, negotiators still praised it and advised members to accept it.

"We can honestly say that this proposal is the best contract we’ve negotiated in our history," the IAM said in a statement.

Aside from the pay rise, the deal offers workers improved healthcare and retirement benefits, and 12 weeks of paid parental leave.

It also includes a commitment from Boeing to build its next commercial plane in the Seattle area if the project is started during the lifetime of the contract. It is not clear when the company will announce its next jet.

The deal also gives the union members more say on safety and quality issues.

"Financially, the company finds itself in a tough position due to many self-inflicted missteps. It is IAM members who will bring this company back on track," the union negotiators said, referring to the crises faced by Boeing in recent years.

Mr Ortberg, an aerospace industry veteran and engineer, took over as Boeing's new chief executive last month.

His appointment came as the firm reported deepening financial losses and continued to struggle to repair its reputation following recent in-flight incidents and two fatal accidents five years ago.