On 21 April Queen Elizabeth II marks her 90th birthday, the first of our reigning monarchs ever to do so, and it will be a very happy occasion, just as her Diamond Jubilee was in 2012. Five years ago there had been a more sombre milestone for the queen’s eldest son, Charles, Prince of Wales. He passed the mark of 59 years spent as heir to the throne set by his great-great-grandfather, Victoria’s eldest son, the Prince of Wales who became King Edward VII in 1901.

The prince will be celebrating his mother’s birthday as enthusiastically as anyone, while oppressed by unmistakable frustration. He’s now 67; the Queen’s mother, the late Queen Mother, lived to 101; when he inherits the throne, he will be well beyond most people’s retirement age.

When or if: as the royal birthday approaches, I’m not the only person wondering whether the monarchy will survive to see the reign of Charles III. Dr Anna Whitelock, director of the London Centre for Public History, said the other day that present support for the monarchy was due to the Queen herself, and warned that the institution might not long outlive her. All those who value constitutional monarchy — which must include the prince — should ponder the very grave difficulties he would face on the throne, difficulties which, it must be said, are largely of his own making.

Such criticisms tend to come from republican malcontents, so let me say that the House of Hanover has no more loyal subject than myself, no stronger adherent to the principles of the Glorious Revolution, the protestant succession and constitutional monarchy. And while I’m not a member of the Prince of Wales’s Party or of the Highgrove Set (if there is one), I’ve always had a soft spot for the prince, as a fellow crank and one more emotionally illiterate Englishman. Some years ago, Alex Beam of the Boston Globe, that sharpest and wittiest of American columnists, listed ‘Twelve Reasons to Love Charles’ by way of answering criticisms aimed at him. ‘He despises the media — who doesn’t? He has failed in love — who hasn’t? He just wants to be left alone — me too. He talks to his plants. I talk to my dog… “The prince’s office had a longstanding reputation for being chaotic” — come see the floor of my office sometime.’ My feelings entirely.

Of course no institution, including monarchy, whether absolute or constitutional, can depend purely on the personal qualities of the inheritor, good or bad. History is littered with rulers whose countries survived them, more or less, and we were particularly brutal in getting through Welsh, Scotch and Dutch rulers until we settled on German princelings just over 300 years ago, for our convenience rather than theirs. One can’t say that the first four Georges knew their place, but the fifth and sixth did. Under Victoria, and for all her gross partisanship, we arrived at that brilliant combination of constitutional or limited monarchy and parliamentary government which Walter Bagehot extolled as The English Constitution nearly 150 years ago.

That settlement has been an incalculable benefit. George Orwell quoted a French journalist who told him that the monarchy was one of the things that had kept England from fascism; and, watching this year as the Americans choose between a ranting boor and an utterly unworthy and untrustworthy woman for their head of state, one can only be grateful for our own system when it works.

It works, that is, when the monarch confines himself or herself to what Bagehot called ‘the right to be consulted; the right to encourage; the right to warn’, if that. It can’t possibly work if the monarch holds and expresses strong views of his own.

In contrast to his mother, the Prince of Wales has never hesitated to tell cabinet ministers, in his famous ‘black spider’ letters, or the rest of us, what he thinks about every-thing from genetically modified crops to grammar schools to alternative medicine. Whether he’s right or wrong is by the way. I happen to disagree with him about fringe medicine and his tenderness towards barbarous Saudi despotism, while sharing his love of the Prayer Book and his distaste for much contemporary architecture. But then I’m not a prince, I’m a journalist, or just a freeborn Englishman with the same right as any other bloody fool to hold forth in the pub. The prince may or may not be a bloody fool, but he’s certainly not a private citizen, and when he dilates on those topics, I’m reminded of what the Duke of Wellington said when explaining why he deplored the practice of soldiers cheering their officers: ‘It comes dangerously close to an expression of opinion.’

Just what a problem the prince has become was spotted some while back by James Lees-Milne. That fastidious conservationist, devout royalist and brilliantly penetrating diarist came to know the Prince of Wales and to like him: ‘A sweet man. Heart bang in the right place.’ But then Lees-Milne went on lethally, ‘Not very clever in spite of praiseworthy intentions. Lays himself open to criticism because he contends with intellectuals and specialists in fields of which he can inevitably have only superficial knowledge.’ To make it worse, the prince has told us that he won’t desist from contending if he inherits the throne, thus demonstrating a complete failure to understand the nature of a constitutional monarch’s role.

If he means it, republicanism will receive a boost it hasn’t known since 1870, when the Prince of Wales was booed at the racecourse after his name cropped up in a lurid divorce case, and when ‘To speak in rude and general terms,’ as Gladstone inimitably put it at the time, ‘the Queen is invisible and the Prince of Wales is not respected.’ A foretaste of our problem had come with his father. Prince Albert was a man of stronger views if anything than the present Prince of Wales. In her recent biography Bertie: A Life of Edward VII, Jane Ridley suggests that Albert’s early death in 1861 might have been for the best, since a Prince Consort with such committed opinions would have fitted awkwardly into the emerging British democracy.

Instead, Albert’s son accepted his modest role. He had regained his popularity by grave illness and recovery from the brink of death — and then by his sheer amiability and charm, until he would be cheered by great throngs at Epsom when he won the Derby for the third time, and in Paris when his visit there in 1903, charming rich and poor in faultless French, undid years of Anglo-French animosity.

Nothing the Prince of Wales has done suggests that he would play that part. And in any case, does he really want to be king? He spends much of his time alone, or in the company of the very likeable second wife with whom he seems to have found the personal contentment that had eluded him. It’s hard to believe that she, in her 69th year, looks forward to gruelling royal duties, or the obloquy which her husband would be likely to incur.

When a Prince of Wales last inherited the throne 80 years ago, he occupied it for less than a year. ‘The Yorks will do it very well,’ Queen Mary said after the abdication in December 1936, and so they did. When the Duke of York was crowned King George VI the following May, Winston Churchill, who had almost wrecked his career over his support for Edward VIII, told his wife Clementine that he had been wrong: ‘the other one’ would never have done, he said, and so he wouldn’t.

That abdication shook the monarchy badly, but it entirely recovered in 1940. From the crowds cheering the King and Queen on VE Day to the far larger numbers than had been foreseen who paid their respects to the Queen Mother when she died in 2002, some of whom were old enough to remember her during the war, to the Diamond Jubilee four summers ago when we remembered that the Queen served as a mechanic in the ATS and that her husband saw action as a young naval officer, the monarchy has drawn on a stock of affection which is not inexhaustible.

Unlike the Duke of Windsor, our Prince of Wales is a fundamentally decent and serious man, maybe too serious for the role he awaits. He possesses a strong sense of duty. Might not it be best expressed by renouncing the throne in advance? It would pass directly to the Duke of Cambridge, an affable man with an engaging young family, who would carry out his duties humbly, while the Duke of Highgrove (or whatever he might be called) could continue harmlessly to bestow on us whatever views he wanted.

God save the Queen! God bless the Prince of Wales! Long live King William V! He must be the best hope, for his dynasty and for all of us.