Defence chiefs have admitted "concern" over a £5bn order for fighter jets that have turned out to be too heavy to take off from warships.
The Joint Strike Fighters are due to come into service in eight years time
The US Joint Strike Fighters are due to replace Britain's ageing Harrier fleet.
The jets, which cost about £35m each, are said to be 3,300lbs (1,500kg) overweight.
The MoD said the plane's engine was heavier than envisaged, but said "problems like this occur in the early stages of complex programmes".
Britain has agreed to buy 150 of the new jets. They are due to come into service by 2012.
The Lockheed Martin planes are 45ft long and 30ft wide.
The fighter jets are said to have greater speed and stealth with which to penetrate the most sophisticated surface-to-air missile defences.
But though the aircraft's engine is widely accepted as the most advanced of its kind, it is far heavier than expected.
If the weight problem is not resolved, the jets will be unable to achieve the vertical take-offs that are the trademark of the Harrier Jump Jet.
Britain is building two new aircraft carriers for the Royal Navy - but the runways will be too short for the jets to take off normally, newspapers have reported.
The carriers would probably need to be redesigned if the jets could not take off vertically, according to Tory defence spokesman Gerald Howarth.
"This revelation furthers the charge that the government is failing to give the proper support to our armed forces," he added.
A spokeswoman for the MoD said: "The weight problem is a concern but problems like this occur in the early stages of complex programmes.
"The projects are being carefully co-ordinated and the problem will be solved in time for the jets to come into service in 2012 as
"From our perspective, these problems do not undermine the programme or our choice of aircraft."