Boeing is offering its staff a 25% pay bump over a four-year contract, 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. A strike can still happen if two thirds of union members support it in a separate vote.

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 union members, it would be the first full labour agreement between the firm and the unions in 16 years.

Although the tentative 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,” said a statement from the International Association of Machinists and Aerospace Workers (IAM).

Aside from the pay bump, the deal offers workers improved healthcare and retirement benefits and a commitment by Boeing to build its next commercial airplane in the Seattle area.

It also gives the union members more say on safety and quality isues.

“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 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.