For pretty much every monarch, it has been a haunting question – how to arrange the royal finances so that one’s heir is provided for before one is 6ft under?

It is testimony to the administrative genius of Edward III – and the Crown’s ability to hang on to its baubles – that the arrangement put in place in 1337 for his eldest son, Prince Edward, by sectioning off the Duchy of Cornwall to fund his lifestyle, remains substantially in place.

But what worked for medieval England’s absolute rulers and successive princes of Wales dating back nearly 700 years is increasingly in danger of being overtaken by the creeping progress of modernity with its irksome insistence on transparency for public institutions.

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Prince Charles – or, more to the point, his sons and consorts – are objects of global fascination and, whether he likes it or not, that includes their finances.

The problem for critics of the current arrangement is that the future king (or queen) of England is funded by a fiefdom over which his subjects have at best only minimal oversight, and they have very little right to be told what it buys and sells or how it goes about its business.

With the sort of confidence that only nearly seven centuries of dominion can muster, the Duchy has a practised response to attempts to find out what exactly it owns.

When The Independent asked repeatedly this week just what it was that the Duchy had decided was such a bargain that it needed to spend £38,385,500 on it 18 months ago, the response was that as a “private estate” it would not divulge the details.

What, it has to be asked, is so confidential about the Prince of Wales’s ownership of a large chunk of a Milton Keynes industrial estate?

The problem for the Duchy is that it is increasingly facing challenges to its cherished status outside the public domain.

The Information Rights Tribunal’s ruling that it is a “public authority” will have sent alarm bells ringing in Clarence House with its implications not only for future disclosure of information, but also for a key finding that its core purpose – funding the future monarch – is a “function of public administration”. Little wonder than an appeal is ongoing.

With Prince Charles doing his best to present a forward-thinking, environmentally-friendly Duchy, the time seems to be right to peel back some of the layers of opacity put in place by Edward III.

It is unlikely to be a rapid process. When Lord Berkeley, putting forward a bill to abolish the Duchy, recently asked to meet the Prince to discuss his proposals, he was told that sadly the royal diary was full.