Delays, infrastructure gaps and personnel shortages to Britain's most advanced warplane programme are undermining the UK's warfighting capability, the public spending watchdog has said.
The F-35 stealth jet used by the RAF is "significantly superior" to all previous UK aircraft, the National Audit Office (NAO) said.
But the NAO said there had been a "disappointing return" on the £11bn spent so far, adding that it estimated plans to expand the fleet would cost more than three times initial Ministry of Defence (MoD) forecasts.
An MoD spokesperson said the programme was within its "approved budget" and there would be two full squadrons of F-35s ready for deployment by the end of the year.
As of June 2025, there are 37 F-35s in service, which are deployed from the Royal Navy's two aircraft carriers.
The F-35s, made by the American company Lockheed Martin, are technologically superior to all previous UK fighter jets and are expected to remain in service until 2069.
But last year, the F-35 fleet's "full mission capable rate" – the aircraft's ability to carry out all of the missions it is needed for – was around a third of the MoD's target.
Plans to equip the new aircraft with weapons that can attack ground targets from a safe range - while ensuring they can retain their stealth characteristics - have been delayed until the early 2030s.
The report from the NAO published on Friday said the capability gained by spending £11bn so far - also higher than initial 2013 estimates - was a "disappointing return".
The £18.76bn the MoD had estimated for the project was "considerably lower" than the NAO's £71bn estimate, the report added.
Gareth Davies, head of the NAO, said: "The MoD now needs to decide where to prioritise its resources to improve capability in a way that maximises the full benefits of the F‑35 programme to the UK."
Prime Minister Sir Keir Starmer said last month that UK spending on national security would increase to 5% of GDP by 2035.
The committment is in line with Nato's target, with countries at this year's summit agreeing to boost defence spending due to "profound" security challenges and a "long-term threat posed by Russia".
An MoD spokesman said: "The National Audit Office's report rightly recognises the world-class capabilities of the F-35 fighter jet, as well as its significant economic benefits - including £22bn of work for UK companies, creating thousands of jobs.
"We also recently announced we will purchase 12 F-35As, supporting 20,000 jobs in the UK, and join Nato's dual capable aircraft nuclear mission."
English (US) ·