It is said to be the most impregnable vault on Earth: built out of granite, sealed behind a 22-tonne door, located on a US military base and watched over day and night by army units with tanks, heavy artillery and Apache helicopter gunships at their disposal.

Since its construction in 1937 the treasures locked inside Fort Knox have included the US Declaration of Independence, the Gettysburg Address, three volumes of the Gutenberg Bible and Magna Carta.

For several prominent investors and at least one senior US congressman it is not the security of the facility in Kentucky that is a cause of concern: it is the matter of how much gold remains stored there - and who owns it.

They are worried that no independent auditors appear to have had access to the reported $137 billion (£96 billion) stockpile of brick-shaped gold bars in Fort Knox since the era of President Eisenhower. After the risky trading activities at supposedly safe …