WASHINGTON (Reuters) – Boeing Co does not anticipate winning approval from the Federal Aviation Administration (FAA) for the Boeing 737 MAX 10 before next summer, according to an agency letter sent on Monday.

Boeing faces a December deadline to win approval of the 737 MAX 7 and 10 variants or it must meet new modern cockpit-alerting requirements, unless Congress extends the deadline.

“With regard to the 737-10, Boeing’s current project plan timeline has the 737-10 receiving an amended type certificate no sooner than summer 2023,” the FAA said in a letter to Senator Roger Wicker, the top Republican on the Senate Commerce Committee, according to two sources who read portions of the letter to Reuters.

The FAA, Boeing and Wicker’s office declined to comment.

(Reporting by David Shepardson; editing by Christian Schmollinger)