Boeing Co (BA.N) does not anticipate winning approval for the 737 MAX 10 before next summer, according to a Federal Aviation Administration (FAA) letter sent on Monday that intensifies concerns about the company’s timeline for deliveries.

Boeing faces a December deadline to win regulatory approval for the MAX 10, which is slightly larger than current 737 MAXs in service, as well as for a smaller variant, the MAX 7. Unless it gains an extension from Congress, Boeing must meet new modern cockpit-alerting requirements that could significantly delay the planes’ entry into service.

“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,” two sources quoted acting FAA Administrator Billy Nolen as saying in a letter to Senator Roger Wicker, the top Republican on the Senate Commerce Committee.

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

Boeing has recently booked major MAX 10 orders from Delta Air Lines (DAL.N), Canada’s WestJet Group and other carriers.

Last week, Wicker proposed extending the deadline for Boeing to win approval for the two new variants until September 2024 and hopes to attach the proposal to an annual defense bill. But it is not clear if Congress would be willing to approve the proposal.

The new cockpit alerting requirements are part of certification reform legislation that was passed in 2020 after two 737 MAX crashes killed 346 people and led to a 20-month grounding for the best-selling plane.

Certifications of planes require extensive paperwork submissions and detailed review of safety assessments by the FAA.

In a Sept. 19 letter to Boeing made public last week, the FAA expressed concerns that the plane maker would not be able to win certification for the MAX 7 this year. Boeing must get approval for the MAX 7 first as the MAX 10 approval is contingent on some MAX 7 documentation, Boeing Chief Executive Dave Calhoun said last month.

The letter added Boeing has not completed all its required assessments and needed to turn in remaining documents by mid-September if it intended to meet the December deadline.

Boeing, which has argued that it is safer to have one common cockpit alerting system for all versions of the 737, said on Friday that it “is focused on meeting all regulatory requirements to certify the 737-7 and 737-10.”

According to one of the sources, Nolen’s letter on Monday said the FAA was unable to provide an estimate as to when certification work will be completed for the MAX 7 or MAX 10.

“We’re committed to diligently and thoroughly reviewing the documentation as it is submitted,” Nolen was quoted as saying.

Nolen also wrote that he supports his team “taking the time they need to fully understand” human factor functions, according to the source.

Calhoun last month told reporters he thought the FAA would approve the MAX 7 this year and said there was still a “chance” of approval for the MAX 10 before the end of the year.