Britain's F-35 stealth fighter jets can only fly a third of their missions thanks to shortages of spare parts and personnel, a damning report by the National Audit Office has found.
The programme is failing to deliver on its promises, with the fleet meeting just a third of its targets last year, the public spending watchdog said.
Its report has found that the 37-strong fleet fell far short of the Ministry of Defence's requirements and exposed how lifetime programme costs could soar to £71billion - nearly four times the MoD's public forecast of £18.76billion.
The MoD has already spent £11billion on the programme, far higher than what it anticipated in 2013.
The 37-strong fleet has been found to have fallen far short of the Ministry of Defence's requirements
PA
NAO findings warned that "combined shortcomings of the global and UK F-35 stealth fighter aircraft programme - including delays, lower-than-expected availability, infrastructure gaps and personnel shortages - are undermining the armed forces' warfighting capability".
Critical operational milestones have also slipped, with full operating capability now expected by the end of this year - two years behind schedule.
Even then, the fleet will have "several gaps against its initial requirements", according to the NAO.
The integration of vital UK-made weapons systems has been severely delayed.
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Britain has committed to purchasing 138 F-35s in total, including a recently announced order for 12 F-35A jets capable of carrying nuclear weapons
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Plans to equip the aircraft with British missiles, including the Meteor extended-range air-to-air missile and Spear 3 medium-to-long range stand-off missile, have been pushed back to the 2030s.
Personnel shortages have also piled pressure on the programme, with a lack of engineers and support staff hampering operations.
The watchdog also raised concerns about spare parts supply and low pilot flying hours, delivering another blow to the fleet's combat readiness.
That £71billion "lifetime costs" figure includes personnel, fuel, infrastructure and other overheads that the MoD omitted from its public figures.
But MoD officials have argued that the department's £18.76billion estimate covers only equipment and support expenses for the initial 48 jets.
Personnel shortages have been blamed for the fleet's failure to get off the ground
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NAO projections are based on the assumption that the full planned fleet of 138 aircraft is expected to remain in service until 2069.
Britain has committed to purchasing 138 F-35s in total, including a recently announced order for 12 F-35A jets capable of carrying nuclear weapons.
Despite the programme's shortcomings, the NAO praised how UK companies manufacture around 15 per cent by value of all F-35s globally, securing an estimated £22billion in contracts for domestic manufacturers.
"The F-35 programme offers significantly improved capability and considerable economic benefits to the UK," said Gareth Davies, head of the NAO.
"But the capability benefits are not being fully realised due to delays, infrastructure gaps and personnel shortages."