USS Wasp Night Ops: An F-35B off on the flight deck of USS Wasp (LHD-1) during operations, a part of Operational Test 1, or OT-1. US Marine Corps The Government Accountability Office released a report on Monday warning the Department of Defense against funding further software updates for the already $400 billion F-35 program until the current software becomes operational.

The F-35 is already operational with the Air Force and Marine Corps, but it runs a limited version of its software, called the 3i block, which only provides 89% of the code required for full warfighting potency.

Meanwhile, as the US keeps buying the jets, Lockheed Martin, the F-35's primary manufacturer, is scrambling to provide 100% of the code with their planned 3F update.

At a conference earlier this year, Lockheed Martin told Business Insider they hoped to have the updated software loaded into the factory and ready to go on new jets by the end of 2017, but the GAO found that claim unlikely.

"Program officials optimistically estimate that the program will need an additional five months to complete developmental testing," the statement reads. But "according to best practices" and "credible estimates," the GAO pegs that number at around 12 months.

And while F-35 program officials admit the delay will cost an additional $532 million, GAO cites $1.7 billion in cost overruns with "approximately $1.3 billion of which will be needed in fiscal year 2018."

One reason the F-35 costs ramp up so quickly, according to the report, is a high level of concurrency between the F-35's development, testing, and procurement. Essentially, all three of those processes take place simultaneously, unlike past defense projects, so a delay in development immediately hurts testing, which then hurts procurement.

Shock collars form around an F-35. Jacek Siminski/The Aviationist

Lockheed Martin officials have told Business Insider that the best way to save money on the program would be a large investment allowing them to buy bigger blocks of F-35s at once, thereby leveraging economies of scale.

But the GAO recommends the opposite — essentially saying the F-35 should not put out request for proposals on the even further out block 4 of F-35 software until they can complete the 3F, base their estimates on historical program data, and resubmit a revised budget to Congress.

As many experts and US servicemembers have told Business Insider, the military awaits the full combat potential of the F-35 for national security reasons as adversarial nations like Russia and China field advanced aircraft and threats of their own.

A Gen 3 helmet is seen in front of a Lockheed Martin F-35 Lightning II fighter jet at Patuxent River Naval Air Station in Maryland October 28, 2015. REUTERS/Gary Cameron

The F-35's block 4 of software promises to open up the full range of combat potential inherent in the F-35, including expanding the jet's arsenal to include nuclear weapons.

President Donald Trump has already weighed in several times on the F-35, with Lockheed Martin's CEO Marillyn Hewson telling Business Insider in March that his involvement in a previous F-35 buy helped save hundreds of millions of dollars.