By David Shepardson

WASHINGTON (Reuters) -Boeing deliveries of its best-selling 737 MAX fell in September to the lowest level since August 2021, the plane maker said on Tuesday, as it continues struggling with work needed to correct a manufacturing defect.

Boeing is inspecting and fixing thousands of holes that were drilled wrong on the 737 MAX aft pressure bulkhead. While new orders improved sharply, the largest U.S. planemaker said deliveries in September fell to 15 737 MAX planes, 10 787s and two 777s, for a total of 27 deliveries.

The monthly number of MAX 8 and MAX 9 deliveries was the fourth-lowest since deliveries resumed in December 2020 after a worldwide grounding following two MAX crashes in 2018 and 2019 that killed 346 people.

The planemaker booked significant new orders in September for 224 planes and reported 10 cancellations. The new orders include 50 787s for United Airlines and 18 787s for Air Canada.

Boeing shares rose 2.6% on Tuesday in mid-day trading.

Investors closely watch delivery numbers, as airplane makers receive the majority of payment for an aircraft when the customer receives it.

Overall, Boeing has handed over 371 planes to customers during the first nine months of 2023, including 286 737s and 50 787s along with 35 other planes. By comparison, European rival Airbus has delivered 488 aircraft over the first nine months, including 55 in September.

Carriers have been placing orders with urgency as Boeing and Airbus backlogs have grown and as jet deliveries have been pushed out toward the end of the decade.

Boeing's gross orders since the start of January rose to 848 in September, or 724 net orders after factoring in cancellations and conversions. Airbus has booked 1,280 gross orders or 1,241 after cancellations.

Boeing's commercial backlog increased from 4,971 to 5,172. Boeing said September was the first month since December 2019 that its official backlog surpassed 5,000.

Separately, Boeing is still awaiting certification of the smaller 737 MAX 7 from the Federal Aviation Administration that has been reviewing the plane for more than a year. The planemaker said in July the first delivery of the 737 MAX 7 had been delayed to 2024.

(Reporting by David Shepardson and Tim HepherEditing by Nick Zieminski and David Gregorio)