But, in May this year, it performed a U-turn, saying it would revert to the F35-B, as costs had increased.

BBC: Joint Strike Fighter planes (file pic)

It says the US - which is leading the F35-B project - is likely to cancel its orders, as Washington struggles with a government debt crisis.

BBC: Joint Strike Fighter planes (file pic)

Mr Hammond insisted the risks associated with the F35-B were "dramatically different now" to what they were in 2010 when there was a possibility it would be cancelled due to technical problems.

BBC: Government in ?100m U-turn over F35-B fighter planes

Abandoning the plan to fit the "catapults and traps" needed by the F35-C to one of the carriers while mothballing the other, opened up the possibility that both could eventually become operational, it argued.

BBC: Joint Strike Fighter planes (file pic)

In its defence review in 2010, the coalition announced that it was cancelling the previous Labour government's decision to purchase jump-jet version of the Joint Strike Fighter (the F35-B) and opt instead for the catapult and trap (F35-C) type, which required a different set-up on deck.

BBC: Joint Strike Fighter planes (file pic)