Gary Slapper: It doesn't sound much, but this is a constitutional watershed

In constitutional struggles, little dramas can have momentous consequences. The curious case of the royal financial memorandum is a headline disguised as a footnote. It is, in effect, a formal farewell to one of the vestiges of monarchical power.

History rarely swings away abruptly on a shocking and unpredictable path. More often, as here, something that has been developing for a while is suddenly made manifest in a particular event.

The legal power and influence of the British monarchy has been slipping away for centuries and this memorandum makes clear and concrete that the ultimate power over the monarch and money is held by a minister from an elected government.

The critical moments in this story of the political and legal decline of the monarch's power arc over the centuries. One key change materialised in "The Case of Prohibitions" in 1607.

During the early seventeenth-century, there had been unease between the King and the courts over who was the final determiner of the law. In 1599, King James VI of Scotland had tried to interfere in a case and was knocked back by the Court of Session in Edinburgh. Things came to a head in the Case of Prohibitions when the King – this time as James I of England – gave judgment in a case and was overruled by the courts. In his judgment, Sir Edward Coke, Chief Justice of the Common Pleas, ruled that disputes were to be tried by legal experts according to law, not the best efforts of an untrained monarch. The courageous Chief Justice said law was the "golden metwand" (measuring rod) against which to judge the claims of citizens.

The reason why the legal measuring rod glowed golden was that law came from a legislative and judicial aristocracy. Today, the measuring rod is golden for a different reason – because the law reflects democratic will.

The British constitution is an amalgam of laws, documents and conventions, and the apparently modest memorandum is a part of that jigsaw. Since it is officially documented in the 2006 memorandum that the legal locus of financial power sits with the minister, it remains to be seen how far the democratic machinery will be used to address the royal budget.

That will be particularly interesting at a time when large parts of the educational, social welfare and police services fall under the axe.

Gary Slapper is Professor of Law and Director of the Centre for Law at The Open University