Members of the British Parliament have described the U.K.’s plan to purchase the Joint Strike Fighter for the carrier strike role as high risk because of the defense ministry’s lack of control of the program.

In a report released Sept. 3, the U.K. Public Accounts Committee (PAC) studied the ministry’s decision to flip-flop its choice of which F-35 variant to operate from the U.K.’s new Queen Elizabeth II-class aircraft carriers. It moved from the short-takeoff and vertical landing (Stovl) F-35B to the conventional catapult-launched F-35C in 2010, then reverted back to the F-35B in 2012.

The original decision to change variants cost £74 million ($115 million), and the decision to change back to the Stovl aircraft is likely to cost a similar amount, the PAC says, although the full costs will not be known until 2014. The ministry believes that, despite writing off the £74 million, reverting to the original decision will help it to avoid £600 million across the 30-year life cycle of the Carrier Strike program. However, the report states that the cost information is still not mature.

The report says the ministry provided its decision makers with “deeply flawed information” on the benefits of changing the type of aircraft to errors such as “omitting value added tax (VAT) and inflation from the costs of converting the carriers.”

Despite defense ministry assurances, the committee says it is not yet “convinced” that it has the aircraft contract “under control,” and describes the whole carrier strike program as high risk.

“Although Carrier Strike is over five years from planned operation, significant technical issues, costs and delivery dates for the aircraft are not resolved,” the PAC report states.

The report also addresses some concerns about the ministry’s definition of interoperability for the carrier strike capability. When the F-35C was selected the defense ministry said there would be interoperability between the U.S. and French navies because they also used catapult and arrestor-based carrier operations. However, when it was realized that the ability to land the carrier variant was more technically challenging than previously thought and the program reverted back to the F-35B, the PAC points out that the emphasis shifted to interoperability with the Italian navy and the U.S. Marine Corps.

Officials are looking toward “Main Gate 4,” a procurement contract milestone due to take place later this year that will not only buy the U.K.’s first squadron but also begin what officials call the transition toward operations. The order will be for around 14 F-35Bs. Around 2018 this squadron should move back to the U.K., where they would begin aircraft carrier trials with the aim of achieving an initial land-based operating capability by late 2018.

The PAC also outlines concerns it has with the planned late entry into service of the Crowsnest carrier-borne airborne early warning (AEW) capability planned for embarkation onto the new ships. The committee says it is not due to be fully operational until 2022. The PAC questioned senior defense officials about why the capability was not entering service at the same time as the ship, and that the lack of airborne-early warning coverage could constrain where the ship could operate.

The PAC says, however, that the defense ministry has “other options” for protecting the carrier, including land-based AEW as well as reliance on allies for the AEW capability.

Aviation Week has reported that Royal Navy officials are now working with the U.K. Defense Equipment & Support procurement agency to try to advance the introduction of the Crowsnest system by two years, in a bid to retain the crew skills developed through the use of the Thales Cerberus radar currently fitted to the Sea King Mk. 7, the current helicopter AEW platform.

Responding to the report, Defense Secretary Philip Hammond said, “As the National Audit Office recognized in their report in May, the MoD [ministry] acted swiftly to switch back to Stovl aircraft as soon as it became clear that the alternative would cost more money. In doing so, we did incur some costs, as the PAC records, of £74 million — but we did so in order to save £1.2 billion, a clear demonstration of our commitment to safeguard taxpayers’ money.”