The F-35 Lightning II’s Block 4 upgrade, which will allow the stealth aircraft to carry nuclear weapons, has been delayed by at least nine months. The F-35 is slated to become the primary nuclear strike aircraft for several US allies. Meanwhile, costs for the program have soared above $1.6 trillion.

According to a new report by the Government Accountability Office (GAO), a congressional watchdog agency, the F-35’s planned Block 4 upgrade has been delayed by nine months, pushing the plane’s full-rate production decision back to sometime between September 2020 and March 2021.

While Block 4 will integrate a number of new weapons into the F-35’s repertoire, such as Naval Strike Missile, the Meteor and SPEAR missiles and several laser-guided bombs, by far the most consequential weapon is the B61 nuclear gravity bomb, which is small enough to fit inside the F-35’s internal weapons bay.

Via the F-35 Block 4, NATO partners who wield US nuclear weapons thanks to nuclear sharing agreements will be able to continue to carry out nuclear strikes. With the Panavia Tornado exiting service with most European partners, a delay in fielding the F-35 Block 4 could leave a gap in NATO’s nuclear capabilities, especially for the Italian, Dutch and Belgian air forces.

However, the GAO report also notes the enormity of the Block 4 upgrade has driven up costs in the already colossal lifetime budget for the F-35. Noting that in 2019 it projected a baseline increase of $8 billion because of Block 4, the GAO stated in its Tuesday report that the update’s development and procurement costs are now estimated to be $13.9 billion and “that the sustainment costs to operate and maintain the F-35 fleet for its planned 66-year life cycle are $1.2 trillion, bringing the total cost of the F-35 program to over $1.6 trillion.”

“The planned $13.9 billion Block 4 effort exceeds the statutory and regulatory thresholds for what constitutes a major defense acquisition program, and Block 4 is more expensive than many of the other major weapon acquisitions already in DOD’s portfolio,” the GAO further states.

“To provide better oversight into Block 4 activities, in 2016, we recommended that the Secretary of Defense hold a milestone B review - a critical point in an acquisition program leading to the engineering and manufacturing development phase - and manage it as a separate major defense acquisition program. DOD did not concur with our recommendation, and it continues to manage Block 4 within the larger F-35 program. We maintain that DOD should manage the Block 4 activities as a separate program.”

The oversight office further advised the Pentagon to continue oversight reports on Block 4 upgrade progress through 2026, even though its budget only provides for updates through 2023.