A pair of Royal Navy aircraft carriers built on the Clyde at a cost of around £6 billion risk becoming 'white elephants' because of defence cuts.

A spending watchdog today warns that the ability of the warships to operate might be "jeopardised".

The National Audit Office found that at least £8.5bn is needed to upgrade the Ministry of Defence's (MoD) estate.

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Without the money, according to the NAO, there is a risk to the UK's military capability.

The report says that the total cost of building the carriers is forecast to total £6.2 bn.

The MoD has been working to ensure that the carriers can sail, including building a new jetty in Portsmouth and dredging a channel deep enough to enable the ships to dock in the port.

"However, because of funding pressures, the Department has not invested in ageing estate assets," the report warns.

"This may jeopardise the carriers’ ability to operate. For example, the cables that supply power to the base and carriers are more than 80 years old.

"They need replacing but there is no funding available to do this."

Liberal Democrat shadow defence minister Judith Jolly said that ministers had "threatened to turn the new aircraft carriers worth £6 billion into white elephants due their incompetence.

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“The condition of much of the Department’s estate is poor and deteriorating.

"How can we, in 2016, be in a situation that we might have to close buildings and facilities because they are falling apart?

"It’s unacceptable and it is high time the government stop looking the other way and properly invest in our troops and defence of the realm.”

Labour’s shadow defence secretary Nia Griffith said the report painted a "shocking picture of mismanagement and underinvestment".

The risk to the new aircraft carriers "represents an outrageous failure to plan for the future", she added.

SNP defence spokesman Brendan O’Hara said: "The findings tell a sorry tale of bad management and bad decisions – leading to uncertainty – and the MoD need to take heed."

Last month the MoD unveiled cost-cutting plans to reduce the size of its estate.

But there was anger in Scotland as Fort George, the home of the Black Watch, was included among the 56 sites earmarked for the axe.

A total of eight sites in Scotland will close over the next 15 years, under the proposals.

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Meg Hillier, the chair of the Commons Public Accounts Committee, said that ministers had been "too narrowly focussed" on reducing the estate.

The department has not yet properly addressed potential risks to military capability, she said.

Defence ministers have said that they will release land for a third of the 160,000 new homes promised by the Conservative Government by 2020.

But Ms Hillier said that ministers needed to get a "better grip" or they would also miss that target.