A heavy cold and a nation shivers. The cold is that attributed, this week, to Britain’s Queen Elizabeth II (her formal titles would take much of the rest of this column). The shivers are those of the political establishment.

Everything of moment in the United Kingdom depends, formally, on the 90-year-old Queen. She legitimizes all laws. She appoints all ministers. Parliamentarians must swear an oath of fealty to her. Ambassadors negotiate in her name, generals fight in it. She is the monarch in more than a dozen former imperial possessions, largely uncontroversially. When, in 1999, on the prompting of a Labour Prime Minister, the Australians tried to usurp her, the move failed, in spite of polls showing only minority support for her. Now, contrary to belief, the polls have risen, to show her popularity at around 60 percent.

Public approval has only grown as she has aged. Shown slowly walking through official ceremonies, even grumpy republicans (of whom I am one, so beware of bias) have to admit to her stoicism and guts. She is by some way the most popular figure in the UK, and the second most popular in the world – after of course, Angelina Jolie (Hollywood royalty still has the edge; it has, after all, a bigger PR budget).

The queen’s formal power is a kind of confidence trick in which almost everyone acquiesces. She does what she is told by the prime minister who comes to her once a week, bows or curtsies, and tells her what the government wants to do - policies on which she can make at most an oblique comment and which she cannot change. A little storm blew up this week as to whether or not she favored Brexit: the BBC political editor said she’d been told she had, but lacking a second source, didn’t broadcast it on her employer’s channels. Denials and no comments have been thick on the ground since; the Queen does not comment, whether she did or not. Brexit steams, or stumbles, ahead.

Her real job is keeping Britain together. Every age group thinks she’s great - the older more than the younger, to be sure - and that the monarchy should carry on into the future - preferably with her at its head. Since that is, however, impossible, the “heavy cold’” has alarmed her country’s real, much less popular, rulers.

Quite soon, a decision must be made - it may have been made already - as to whom the succession will go. It is on paper simple: to her eldest son, Charles, Prince of Wales, 68 last month. But here’s the rub. Though less unpopular than he was during the divorce from Princess Diana in 1996, and after her death in 1997, his approval ratings remain mediocre, and even admirers think he should abjure the throne for his elder son, Prince William.

Charles is a man of opinions - on the environment, on architecture and on government support for his many charities and causes - opinions he presses insistently on governments, as shown by his letters, released under the Freedom of Information Act. If, as king, he continues in this, both he and his institution will suffer for it.

Prince William, 34 last June, has, by contrast, no known opinions on public issues. He has a wife, Catherine, from a non-royal background; they have two young children, a boy, George and a girl, Charlotte. The whole family is photogenic and seem charming, though William is balding early. He trained in the Royal Air Force as a helicopter pilot, and works full time as a pilot for the air ambulance service after his short spell in the RAF. That mix of military service and aiding the sick is a potent one.

From a public relations point of view – one of the most influential in monarchical conclaves – he is a gift, in spite of the blunders said to have come from ignoring his PR team’s advice. His father, though, poses the real challenge.

If Charles succeeds – it’s still more likely than not – then the monarchy ceases to be an unquestionable asset, and becomes a zone of nervous image management. If William succeeds, Charles’ disappointment may burst out in public explosions. Even if not, the burden on William of carrying on a tradition so long occupied by his grandmother would be heavy upon one whose political antennae are untested, in a country whose domestic and international frameworks are shifting and fragile.

So popular has the queen been that even the solidly republican Scottish National Party, which provides Scotland’s regional government and nearly all of its representation at Westminster, cannily shifted towards a royalist position. But neither the son nor the grandson would command the same grip on Scots’ sentiment. That, coupled with Scotland’s vote to remain in the European Union, could convince waverers that the independence the SNP exists to attain was worth the economic risk.

Elizabeth had the power of the powerless – which in her case was world fame. Everyone who was anyone wished to meet her: and in her decades of rule, she met almost every world leader, most of whom she has outlived. To meet her was to touch history, a tourist destination for the global elite.

A dis-united Britain would be a weaker member of the Western alliance. Its weight as a member of the United Nations Security Council would be lessened; it would be out, or on a path out, of the EU and its international reputation as a supporter of liberal politics, trade and economics, would be further damaged.

It would, unwillingly, have dropped the pilot – a pilot who was not supposed to direct the course of the ship of state, but made its progress more stately. A former foreign secretary, Douglas Hurd, coined the phrase in 1993 that the UK “punched above its weight” in the world, especially in military operations. In that boxing metaphor, the queen was an off-the-ring trainer, a symbol of what the military was fighting for, lending a human face and example to the abstraction of the oath of loyalty.

A diminution of Britain on the passing of Queen Elizabeth will be hard to avoid. It’s unlikely to be staunched by whoever is her successor. Only with the transition to a republic, might a new energy be found, a new character be formed. But that is the anti-monarchical propaganda I warned you about. And like all great schemes of change, who knows if it would work?