F-35 fighter jet development hampered by many flaws
A list of serious flaws has been found in a jet fighter in development for 15 years.
The F-35's ejector seat "failed to meet neck-injury criteria" and the jet had a "limited ability to respond to threats", the US defence department has said.
The jet's development, by Lockheed Martin, has already cost billions, partly funded by the UK.
Previously, its UK planned deployment date was put back from 2012 to 2018.
"It's one of the most delayed and problematic fighter programmes in history," said Justin Bronk, a military analyst at the Royal United Services Institute (Rusi).
"While the aircraft could be operationally deployed in its current state, pilots would not be able to undertake such high-g force turns as planned with a lot of fuel on board because of limitations in the flight control software which will take time to overcome.
"If this sort of news keeps coming out in terms of continued problems with the testing... it's potentially embarrassing," he told the BBC.
But he added the aircraft should still be very advanced and capable when complete.
"They will make this thing work because they have no choice, there's no alternative," he said.
"They will chuck whatever money they need to at it."