Abandoned plans to switch the type of fighter aircraft used on the Royal Navy’s new carriers will cost taxpayers £74m, according to the National Audit Office.

Defence secretary Philip Hammond last year 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 proposals set out in the 2010 Strategic Defence and Security Review, the coalition had intended to switch to the F-35C carrier variant of the aircraft, even though it meant mothballing one of the two carriers on grounds of affordability.

But the costs of fitting the necessary catapults and arrester gear to the carriers had more than doubled to £2bn.

The National Audit Office (NAO) said although the department acted quickly once it had realised the problems with switching, the decision made in the defence review was based on ‘immature date and flawed assumptions’, and the subsequent work cost about £74m.

Amyas Morse, the head of the NAO, said: ‘It is good that the Ministry of Defence acted promptly, once it became clear that pursuing the option to buy the carrier variant aircraft would cost a lot more money and add another three years to the whole programme.

‘But to achieve value for money in this project, the department will have to manage significant technical and affordability risks, and be consistent in sticking to the present plan.’