LONDON — The U.K. government’s decision to change its approach on buying aircraft carriers and Joint Strike Fighters has added risks to the long-term program that are still not fully understood months after those changes were spelled out in the Strategic Defense and Security Review (SDSR), the National Audit Office warns.

Although the audit report states that the move from the F-35B short-takeoff-and-vertical-landing design to the F-35C catapult-launched version results in the U.K. buying a more capable aircraft, it notes that the switch “has introduced six new areas of risk” and that the Ministry of Defense “has not yet generated quantitative assessments of the risk impacts and, consequently, has not fully costed them or put funded mitigation plans in place.” That is not expected to be the case until the end of the year.

The six risk areas include how the F-35C performance, optimized for U.S. carriers, will fit on the U.K. design from a safety and airframe-life perspective; providing air-to-air refueling as a backup in case carrier landings are impeded; lack of a formal U.K. requirements statement for the F-35C; cooperation with the U.S. Navy to build up flat-deck carrier operational experience; and lack of a U.K.-operated F-35C in the initial test program. The U.K. has bought three F-35Bs, a decision that predates the SDSR procurement shift, although efforts are now under way to convert one of those to a C-model.

Margaret Hodge, who chairs Parliament’s Committee of Public Accounts, warns that “there are new cost and value-for-money risks which have yet to be quantified and which in the current financial climate are clearly unaffordable.” And, she fears, “the carriers may once again be a victim of the need to balance the books in the short term.”

On the positive side, auditors note that the move to the F-35C allows the ministry to cover a capability gap brought about by last year’s decision to cancel the Deep and Persistent Offensive Capability requirement. Canceling that requirement generated a ₤1 billion ($1.6 billion) long-term savings, but a capability gap as long as the F-35B was being purchased. The F-35C can now cover that gap.

The NAO report also spells out the far lower aircraft sortie rate under current plans to only have 12 F-35s at sea using one aircraft carrier, with the second carrier being bought but maintained at a yet-to-be-defined extended readiness level. Rather than being able to generate 73 sorties per day using 36 aircraft, the carriers will likely only be able to generate 20 per day. Furthermore, average at-sea time is coming down to 150-200 days from 435 days if two carriers were built and made fully operational. The NAO says the sortie rate could increase again, since the carriers are being configured to accommodate 36 JSFs if such a purchase decision is made.

To operate with only one carrier properly operational, the U.K. plans to work closely with the French and U.S. navies, but the NAO report warns that “the feasibility of flying the JSF carrier variant from the French carrier and the French aircraft (the Rafale) from the United Kingdom carrier is as yet unclear.”

Auditors also point to risks with using the Electromagnetic Aircraft Launch System, in part because of risks in the overall U.S. program, and because the U.K. version would feature a two-rail rather than a four-rail system.