FILE PHOTO: A Boeing 737 MAX sits outside the hangar during a media tour of the Boeing 737 MAX at the Boeing plant in Renton, Washington December 8, 2015. REUTERS/Matt Mills McKnight

(Reuters) - Shares of Britain’s Senior Plc, whose aerospace unit counts Boeing Co as its biggest customer, fell 5% after the U.S. planemaker said it would suspend production of its best-selling 737 MAX jet.

Boeing said on Monday it would suspend production of the jets in January, its biggest assembly-line halt in more than 20 years, as fallout from two fatal crashes of the now-grounded aircraft drags into 2020.

Senior manufactures parts including airframes and engine build-up tubes for the MAX jets.