I suppose we should all be grateful to the Prince of Wales. Until he opened his mouth in Abu Dhabi, I doubt if any of us realised how much better off we would be eating a Big Mac than wolfing down one of his own Duchy Originals organic Cornish pasties.

On a tour of a diabetes centre, Prince Charles asked a nutritionist: “Have you got anywhere with McDonald’s? Have you tried getting it banned? That is the key.”

Er, no. You see, old chap, if you’re worried about nutrition, it turns out that a Big Mac, according to figures published yesterday, has nothing on one of your own comestibles. A Duchy Originals organic Cornish pasty has 264 calories per 100g, and a Big Mac only 229 calories; a Duchy Originals pasty has 5.5g of saturated fat, a Big Mac just 4.17g.

You have to hand it to the Prince. There aren’t many people who can manage to be a loudmouth, a danger to the constitution and a buffoon all at the same time. Most of us can manage two of the three. Prince Charles is unique in getting the hat-trick.

That he is wrong, or at the very least a hypocrite, about Big Macs is, however, the least of it. Even if he was right — and by the law of averages he will surely be right about something, one day — his behaviour is an outrage against the constitution and undermines what little credibility the institution of the monarchy has left.

The Prince of Wales has shown over the years that he is simply a loudmouth who cannot resist shooting his mouth off when an opportunity arises. And as he is the heir to throne, such opportunities arise at will.

Whether it’s the supposedly deplorable state of modern architecture (a matter of taste), the efficacy of alternative medicine (voodoo, not science) or the superiority of organic produce (an assertion with no evidential basis), Prince Charles appears to be a man of limited intellect, but to be nonetheless desperate to share the produce of that limited intellect with the rest of the country.

You may think the accusation of loudmouthery somewhat hypocritical from a newspaper pundit. But there is one crucial difference, which goes to the heart of Prince Charles’s position. Newspaper columnists — and politicians — get paid to share their views with the country. The Prince of Wales gets paid to do the opposite: to share his views with no one. That’s what comes with being heir to the throne.

He may not like it. He may be convinced that we need to hear his wisdom. Tough. His job is to keep shtoom like his mother has done for 81 years. And if he doesn’t like it, there’s a simple solution. Stop being HRH, stop being heir to the throne and join the real world. Step forward, Charles Windsor, La-La-Land Party candidate and rent-a-quote pundit.

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