The full costs of scaling back Britain's aircraft carrier programme are still not fully understood by the Government more than a year after the changes were made in the Strategic Defence and Security Review (SDSR), a cross-party committee of MPs has said.

The cash savings from the revised plan will only be £600 million and there remained "considerable uncertainty" about the cost of modifying one of the new ships to accommodate a different kind of fighter jet, according to the Commons Public Accounts Committee (PAC).

That would not be known until December 2012, the committee said. It accused the Ministry of Defence (MOD) of focusing on "short-term" affordability in drawing up options for the SDSR to cover a £38 billion black hole in its budget.

Under the SDSR plans set out in October 2010, the construction of the two Queen Elizabeth class carriers ordered by the previous Labour government is going ahead.

However, one of the ships will be mothballed to save costs and the other will be converted to operate a cheaper Joint Strike Fighter, rather than the short take-off, vertical-landing (STOVL) version planned by Labour.

There will be no carrier aircraft capability between 2011 and 2020.

In a report critical of the long-term value for money of the Government's revised carrier programme, the PAC said: "To convert the ship has changed the profile of risks and costs, and the costs are not yet fully understood. The switch from the STOVL variant of the Joint Strike Fighter to the carrier variant has reduced the technical risks associated with the STOVL aircraft.

"But the costs of converting the carrier for the carrier variant aircraft will not be known until December 2012, leaving the project at risk of cost growth and slippage, and there are new technical risks and challenges integrating the new aircraft with the carriers."

Shadow defence secretary Jim Murphy said there was "a gaping hole" in the Government's credibility on defence.

"Yet again we have a respected body giving a damning assessment of the defence review, which was driven by short-term cash savings, not strategic need, and limits Britain's ability to project power," he said.