The UK National Audit Office (NAO) has detailed the challenges faced by the Royal Air Force (RAF) and Royal Navy (RN) in fielding the Lockheed Martin F-35 Lightning II, while projecting that the nation’s through-life spend on the stealth fighter programme could potentially top £70 billion ($94 billion).

Published on 11 July, the spending watchdog’s report – titled The UK’s F-35 capability – summarises the procurement to date as having fallen short of expectations.

The UK has so far received 38 short take-off and vertical landing F-35Bs, from a total commitment for 48. One of those jets was written off in 2021 when it fell from the ski-jump ramp of the aircraft carrier HMS Queen Elizabeth into the Mediterranean Sea.

The NAO says the MoD expects to have taken another three jets by the end of this year, with the remaining aircraft from its current orders to be accepted by April 2026.

While the UK government recently announced its intention to modify its planned next order to total 15 B- and 12 A-model F-35s, the NAO highlights that its most recent order commitment was made in 2016 and has been the subject of delivery delays.

Meanwhile, operational use of the type has fallen significantly short of planned levels. The NAO says that in 2024 the fleet’s achieved full mission capable rate “was approximately one-third of the MoD’s target”. It has not disclosed the actual percentage figure due to “national security reasons”.

The auditor attributes the shortfall to factors including “a UK shortage of F-35 engineers and a global shortage of F-35 spare parts”, and insufficient “cyber specialists, pilots and flying instructors”.

Pointing to the F-35B’s specialist carrier strike role, it adds: “The F-35 force is also perceived by some Royal Air Force personnel as a comparatively unattractive posting because of long carrier deployments.” It also highlights “the poor quality of some domestic accommodation and amenities at RAF Marham [in Norfolk]” that act as a further disincentive.

Such influences “have resulted in fewer flying hours for pilots than the MoD wants”, it says, citing a total of 3,106h as recorded in 2024.

“The MoD originally set a requirement of 10 hours per pilot per month. Even after [it] reduced the requirement to approximately 7.5 hours per month to account for other duties required of pilots, this number of flying hours was insufficient to meet requirements for all pilots in post as of March 2024,” the report reveals.

“The level of overall UK capability is currently lower than the MoD intended it would be by now at the time of its 2013 business case [investment decision], and will be for several years,” it says.

And despite requiring “a significantly lower ratio of pilots to aircraft than by the global F-35 programme”, it says: “The MoD is forecast to have slightly fewer F-35 pilots than required until 2029‑2030.” Measures intended to address other personnel shortages also are not expected “to fill gaps until 2028”, it notes.

Regarding the type’s combat capability, the report highlights: “The F-35 does not have a standoff weapon to attack ground targets from a safe range, which will impact its effectiveness in contested environments.

“The UK is dependent on the Joint Program Office (JPO) providing laboratories and aircraft to test missile integration, but the integration of new weapons is delayed as part of the wider Block 4 upgrade delays,” it states. That means that the MBDA Spear 3 air-to-surface missile and Meteor beyond-visual-range air-to-air missile – once planned for availability from December 2024 – has now slipped to “the early 2030s”.

The NAO highlights “delivery of a key component from the US” for Spear 3 and “low priority given to Meteor by the global [F-35] programme” as further factors.

Noting that the current Raytheon Systems Paveway IV precision-guided bomb “lacks the range to be an effective standoff weapon”, it reveals: “To acquire a more capable interim air-to-surface weapon the UK F-35 programme has requested funding for Small Diameter Bombs.” However, “The MoD has yet to provide this funding.”

Meanwhile, a full operational capability declaration for the F-35 fleet is due to be made late this year, although the report notes that this milestone – representing the readiness of the RAF’s 617 Sqn and the RN’s 809 NAS, with 12 available jets each – will occur two years later than originally planned.

The UK’s ability to support its fleet in-country has also been affected by a decision to delay investment in an aircraft signature assessment facility (ASAF). In 2021, the MoD opted to halt construction work for three years, to make a saving of £82 million, “even though this would lead to higher costs in the long term”.

According to the watchdog, “This [facility] is necessary for the MoD to assure itself that the aircraft retains its stealth characteristics [which] can become eroded over time and require a cycle of checking for, and repairing, any damage.

“The MoD is now considering building a joint ASAF for the F-35 and the Global Combat Air Programme,” it says, referring to the sixth-generation fighter development between the UK, Italy and Japan. “In the meantime, the MoD is planning to send two aircraft a year to an ASAF facility in Italy, but this will not provide the scale or freedom of action it wants.”

As calculated by the MoD, total UK spending on the F-35 had reached £9.35 billion by the end of March 2025. The NAO estimates that this stands at £11 billion once also factoring in expenses such as “personnel, infrastructure, training flights, other overheads, and earlier sunk costs”.

The NAO notes that an expectation “that F-35 would be comparable to or cheaper than fourth-generation fighters such as [Eurofighter] Typhoon” has been proven to be “over-optimistic”.

“The capacity achieved for the estimated £11 billion spent to date is a disappointing return so far compared with MoD plans, even if other programme benefits have been significant,” it says. For example, UK companies have to date secured contracts against the programme valued at £22 billion.

Estimated figures regarding a total planned programme of record acquisition of 138 F-35s and use of the type through 2026 are almost £57 billion, the MoD says, although the NAO’s broader assessment method suggests “a full whole-life cost of £71 billion”.

“The MoD should ensure that [future] F-35 investment requests receive scrutiny based on the value of the specific request being made rather than the value of the total F-35 programme,” the NAO says in its recommendations.