Boeing Co. is set to announce a major order from Ryanair Holdings Plc for about 150 of its largest 737 MAX aircraft, an important endorsement from a longterm customer that adds crucial sales momentum for the jet intended to counter Airbus SE’s popular A321neo.

The deal, set to be announced as soon as tomorrow, would be the 737 MAX 10 model, with the possible addition of 50 options, according to people familiar with the deliberations, who asked not to be identified discussing confidential negotiations.

Boeing declined to comment, while Dublin-based Ryanair wasn’t immediately available to comment. Reuters reported earlier today that the discount specialist is close to signing a major aircraft order with Boeing for more 737 Max aircraft, citing two unidentified people familiar with the matter.

--With assistance from Julie Johnsson.