Boeing strike: Workers overwhelmingly backed the walkout

Almost 95% of the union members who voted in the ballot rejected the pay deal, with 96% backing strike action.

“Our members spoke loud and clear tonight,” said Jon Holden, president of the International Association of Machinists and Aerospace Workers District 751.

“We strike at midnight,” he added.

Earlier this week, union representatives advised members to support the tentative deal.

As well as a 25% pay rise over four years, the preliminary agreement that workers rejected included a commitment from Boeing to build its next commercial plane in the Seattle area if the project started during the lifetime of the contract.

The union initially targeted a number of improvements to workers’ packages, including a 40% pay rise.

Mr Ortberg made a last-ditch plea to workers before the vote, warning that a strike would put the company’s “recovery in jeopardy”.

The current contract between Boeing and the unions was reached in 2008 after an eight-week strike.

In 2014, the two sides agreed to extend the deal, which is due to expire at midnight on Thursday.

“It’s never a good time for a strike, at least from the perspective of management, the current situation makes it even more problematic,” said Greg Waldron, Asia Managing Editor at aviation news website FlightGlobal.

“Still, a great deal will depend on how long the strike lasts. Airline CEOs with 737 Maxes on order will be watching this closely,” Mr Waldon added.

In July, Boeing agreed to plead guilty to a fraud charge and a criminal fine of nearly $244m (£190m) in connection with the fatal crashes of two of its 737 Max planes more than five years ago.

It is also facing other lawsuits and probes after a mid-air blowout in January of a door plug on a new plane flown by Alaska Airlines.

On top of mounting financial losses, the plane maker has had to slow down its assembly lines due to a 737 Max production cap imposed by the US Federal Aviation Administration.