Former princes meet presidents all the time. It’s a crucial element of the day job. The key for royals, as for politicians, is timing. And the encounter between King Charles and the European Commission president Ursula von der Leyen was seriously mistimed.

Whatever the spin – and there’ll be plenty – the photograph of the two together can be interpreted as the Head of State endorsing the Windsor Framework. Such an endorsement is something Charles should run a million miles from, given the ink isn’t yet dry on the paperwork and the detail of the deal has yet to be examined forensically by all interested politicians.

I have to assume that vanity played a part in today’s regal debacle – a desire to appear important

The warning signs were there when those responsible for setting up the audience at Windsor Castle couldn’t even agree on its genesis. According to palace officials, the King was acting on government advice. No. 10 insisted it was fundamentally a decision for our constitutional monarch. A late entry to the pass the ‘whose great idea was this’ parcel was the suggestion that the EU chief herself had requested a cosy chat with Charles over tea and Duchy Originals.

Governments, with their backs against the wall, will always seek to burnish their credentials with a touch of Windsor glitter. It’s the responsibility of the relevant royal and those around them to judge when this isn’t appropriate. One of the reasons Queen Elizabeth had such an unblemished reign is that her advisers knew how to deftly avoid such pitfalls. They’ll have been watching today’s developments aghast, with their hands over their eyes from behind the sofa.

I have to assume that vanity played a part in today’s regal debacle – a desire to appear important, statesmanlike, playing a role at a crucial time. A son keen not to be always defined by the way his mother did things. But make no mistake, this was a very serious error of judgement by King Charles and his advisors.

With one handshake and the click of an attendant camera, he abandoned his unifying role and entered a political fray where the Brexit dust has yet to settle.

The hope has to be that he’ll depart the fray bruised and wiser, with his advisers. Not all of them though. Someone’s head will roll.