Royal Navy pilots have been sent to America to train on catapult-launched fighter jets - prompting speculation the cheaper aircraft will be bought for the new Portsmouth-based carriers.
Two Queen Elizabeth class carriers are due to have 150 F-35 Joint Strike Fighter jets onboard.
The Lockheed Martin-built fighter planes will have a short take off and vertical landing - the same technique used by the Harriers.
But the estimated cost has risen dramatically over the past nine years to 262bn.
The MoD has confirmed a group of 12 Royal Navy pilots will undergo training with the US Marine Corps over the next eight years - including training on the catapult-launched F-18 fighter jets.
It's thought that as part of the government's defence spending review, the MoD is looking at other options and could buy the cheaper, catapult-launched version of the F-35 instead.
The MoD has denied it was looking to axe the jets.
But recently power conversion specialist, Converteam, said it had been awarded a 650,000 contract from the MoD to develop an electromagnetic catapult system suitable for the new aircraft carriers.
Portsmouth South MP Mike Hancock, who sits on the Commons defence committee, said: 'It's been a long time since British ships had catapult launched fighters so pilots would need training if that's the direction we're heading. There are a lot of unanswered questions.
'It's not certain whether the Joint Strike Fighters will be up for the job and they're incredibly expensive at a time when the MoD are looking to reduce costs.
'We might even have the situation where we have aircraft carriers built but no aircraft to put on them.'
The contractual decision on what type of F-35 to buy does not have to be made until early in 2011.
Carrier builders BAE said the ships wouldn't require a significant redesign as they've been designed to have flexible platforms.
But an MoD spokesman said 'it would be wrong to assume' that it had a preference for a catapult-launched version of the F-35.
The carriers are due to come into service from 2016 and 2018
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