The crown is the embodiment of English law. So it is pretty hard to bring a case against the monarch, and almost as difficult to bring a case against the monarch’s heir. Very occasionally, however, it happens. And the courts have made such a ruling today, permitting the publication of Prince Charles’s letters to ministers. The judgment doesn’t shatter the state in the way that the decision to execute Charles I did in 1649. But it opens a surprising number of similar questions.

Prince Charles memos: supreme court rules in favour of the Guardian - live Read more

The fact that Prince Charles likes to write to government ministers expressing his strong views is no secret. It is a central part of the engaged, in some ways constitutionally destabilising, reality of the ageing and anguished heir to the throne, and was well brought out most in Catherine Mayer’s recent biography.

The content of those letters is, however, a total secret. It has been zealously guarded by the recipients of the letters themselves, and over the last few years, by the full might of the British state and government, as Whitehall has fought every step of the way to stop the Freedom of Information Act disclosure of the letters to Rob Evans of the Guardian.

Today’s decision by the supreme court to uphold publication is therefore a big win for Evans, the Guardian and freedom of information. By the same token it is also a big defeat for Charles, the recipients of his letters and the British state. But it may not yet be the end of the matter.

Today’s judgment was widely expected by lawyers. That’s because, as the judgment itself makes extremely clear, the FOI process was properly used and the FOI upper tribunal, which has the status of the high court, conducted a very thorough hearing before it ruled in 2012 that the letters should be disclosed. Neither the court of appeal, which upheld the tribunal’s decision, nor the supreme court, showed any appetite for overruling that decision. So another victor today is the freedom of information legislation and its machinery.

The case has become much more fraught because of the efforts of the former attorney-general Dominic Grieve, on behalf of the government (in other words on behalf of the crown), to try to exempt the prince’s correspondence from disclosure on the grounds that he, the attorney-general, had the power to override the upper tribunal when he issued a certificate protecting the letters.

So, although the larger and more newsworthy constitutional clash in this case is about whether the law should apply to the heir to the throne – and thus about the wisdom or not of the prince’s way of lobbying ministers – this wasn’t the uppermost issue for the court today. For the court the key issue was whether Grieve was entitled to override the tribunal. And it was clear in its judgment that he was not.

This will not be the end of the matter, because of two things. First, the Guardian has to go back to the FOI process and get the prince’s letters put into the public realm. That’s likely to happen fairly soon – but it explains why the supreme court’s ruling doesn’t mean we can all read the letters today. But it is also possible that the government will have hatched a legislative strategy to minimise the possible disclosure of Charles’s doings and to make absolutely sure that the impact of today’s decision is decreased.

In the end, though, this case is about two much broader issues. The first is about the position of the heir to the throne in the UK constitution. The government tried to argue that, although Charles is not himself the monarch, he is in training for the role, so the same sort of confidentiality must apply to his dealings with government as applies to that of the Queen. The problem with this argument is that the idea of training for monarchy is a dubious one. It is not a training regime that ends in any form of qualification. Charles will become king through the law of succession, not because of any fitness for office. And how long does a period of training last? Charles could be said to have been “training” for the last 66 years. This argument is bogus from beginning to end. But that doesn’t mean that there will not now be an attempt to legislate something of this sort to protect not just the heir to the throne from FOI requests but all the other royals too.

The second issue is the particular character of Charles. He is a meddler. He wants to play a role beyond the ceremonial. He clearly believes, as Mayer’s book reminds us, that he can be an active part of the government of Britain. Although his vision of himself seems to owe a lot to Prince Albert in the 19th century, a better comparison is the 18th-century one. He wants to have a prince’s party in the governance of the state. It is a vision suited more to the era of George III than of Charles III. In his mind, that active part is a benign one, putting forward arguments on issues that he somehow thinks it is appropriate for him to pursue. But this is his well-paid and irreconcilable tragedy. Ours is a parliamentarily controlled monarchy. It’s what our ancestors fought the civil war and the glorious revolution to create. The crown may still embody the law. But in this system there can be no such thing as a “useful prince”.