Development testing of the Lockheed Martin F-35 could be delayed by 12 months and cost another $1.7 billion, the US Government Accountability Office (GA0) warns in a new report published on 24 April.

In a report submitted to the US Congress, the GAO says that the F-35’s government managers at the joint programme office (JPO) have adopted an “optimistic” estimate for a five-month delay and $532 million cost overrun to complete Block 3F software, the fifth and final software release to support the 15-year-long system development and demonstration phase of the family of stealth fighters.

GAO’s analysis based on historical data suggests Block 3F testing won’t be complete until May 2018, or 12 months later than currently scheduled. The GAO’s anticipated or cost growth of $1.7 billion would raise overall development programme costs to $56.8 billion, $22.4 billion higher than the original budget at contract award in October 2001.

If the GAO’s delay materializes, it could drive knock-on delays for initial operational capability of the F-35C variant in Fiscal 2019, starting Block 4 follow-on modernisation in FY2018 and the JPO’s plan to start buying F-35’s in economic order quantities beyond annual procurement lots.

By the end of 2016, the US and partners had ordered a total of 285 F-35s, including 68 by international partners or foreign governments, through the first nine lots of low-rate initial production.

The development phase is scheduled to complete next month, but 10% of the test points remained incomplete as of December.