The U.S. Government Accountability Office, in a study released Monday, sounded the alarm about software development for the high-tech, radar-evading jet fighter.
The budget watchdog warned American law-makers that the separate computer programs that verify the aircraft's "basic operation" and "warfighting" capabilities, such as the firing of missiles and the dropping of bombs, could be delayed by 13 months.
A few dozen of the stealth fighters are already in service, but are mostly equipped with training and other rudimentary software that allows for testing.
The GAO says the current programs have "limited capability" and there's been "a need to fix problems and retest multiple software versions."
The manufacturer, Lockheed Martin, is supposed to come up with improved software that will allow the U.S. Air Force to begin using the F-35 in initial operations by July 2015, but the accountability office says that date is in question.
In Canada, the Harper government has yet to say whether it will stick with the F-35 program — or launch a full-blown competition to replace the country's aging fleet of CF-18s.
The plan to buy 65 of the ultra-modern jets was put on hold after an auditor general's report, almost two years ago, accused both National Defence and Public Works of failing to do their homework and low-balling the enormous lifetime cost.
Subsequent independent reviews have said it would cost Canadian taxpayers up to $44 billion over 40 years to buy, maintain and operate a modest fleet of F-35s — much more than the $16-billion price tag the Harper government initially floated.
The government has yet to buy any aircraft.
The program became the focus of opposition and public outrage, but has since dropped off the political radar, leading some in the defence industry to speculate the Conservatives want the issue to go away until after the next election.
The U.S. budget watchdog's report comes on the same day as Lockheed Martin was celebrating a decision by South Korea to buy the F-35 to replace its existing fighter jet fleet.
In order to keep the program on track, the Pentagon "will have to increase funds steeply over the next five years," said the accountability office.
"Annual funding of this magnitude clearly poses long-term affordability risks given the current fiscal environment," said the review.
"The program has been directed to reduce unit costs to meet established affordability targets before full-rate production begins in 2019, but meeting those targets will be challenging as significant cost reductions are needed."