The Ministry of Defence spent £100 million on now-abandoned plans to switch the fighter aircraft for the Royal Navy's new carriers, Defence Secretary Philip Hammond has revealed.
As ITV News' senior political correspondent Chris Ship reports, in a major policy U-turn, Mr Hammond said the National Security Council decided to revert to plans by the former Labour government to acquire the jump jet version of the US-built F-35 Joint Strike Fighter:
Under plans set out in the 2010 strategic defence and security review (SDSR), the coalition had intended to switch to the more capable F-35C carrier variant of the aircraft.
But Mr Hammond told the Commons that the costs of fitting the necessary catapults and arrester gear, "cats and traps" - needed for take-off and landing - had more than doubled to £2 billion.
At the same time delays to the programme meant that it would not now be operational until 2023: three years later than planned.
The Defence Secretary said reverting to the jump jet would also open the possibility that both carriers could become operational and that one would not have to be mothballed as planned under the SDSR.
Pressed by shadow defence secretary Jim Murphy, he said £40 million to £50 million had been spent on the assessment and design work for fitting the cats and traps and that the MoD could also face "exit" payments to contractors in the US.
He later told BBC Radio 4's The World at One that the final bill was likely to be "something in the order of £100 million".
Defence sources said the conclusion that the F-35B represented the best option had been "somewhat unexpected".
At the time of the SDSR, David Cameron told the Commons that the previous Labour government had "got it badly wrong".
He argued the F-35C had a longer range, carried a bigger weapons payload and, unlike the F35B, would be fully interoperable with the US and French navies whose carriers were fitted with cats and traps.
But defence sources said the F-35B was able to carry all the weapons available in the UK arsenal, while most operations involved air-to-air refuelling, "mitigating" the need for a longer-range aircraft.
Mr Hammond said the choice of the F-35C in the SDSR had been "the right decision based on the information at the time".
However, Mr Murphy said Mr Cameron had overruled the advice of his officials that it was a "high risk and high cost" option and called on the Prime Minister to apologise.