Just five years into Elizabeth II's reign, the young Queen was told she was putting the British monarchy at risk of being toppled.
The Queen of the Realm, Head of the Commonwealth and Defender of the Faith was called a "pain in the neck" and a "priggish schoolgirl", who was being led astray by her "tweedy" and "snobbish" courtiers.
"The Monarchy will not survive, let alone thrive, unless its leading figures exert themselves to the full, and with all the imagination they and their advisers can command," the editor of the National and English Review, Lord Altrincham, wrote in 1957.
The opinion piece, which caused a sensation, warned Elizabeth that she must "transcend" race and class if she had any hope of remaining relevant to her subjects in Britain and around the Commonwealth.
"When she has lost the bloom of youth the Queen's reputation will depend, far more than it does now, upon her personality," Lord Altrincham warned.
"As yet there is little sign that such a personality is emerging."
The Queen living in a king's shadow
The article was potentially devastating for the 30-year-old monarch, whose father and predecessor George VI was beloved by the British public. While her coronation was a joyous occasion, the shine on the new Queen was starting to wear off by 1957.
The empire built by her ancestors was slowly crumbling, and the loss of the Suez Canal to Egypt in 1956 dealt a crushing psychological and strategic blow to Britain's status as a global superpower.
The sun was setting on the British empire. What need was there for an empress?
But the 40th person since William the Conqueror to wear the crown was determined not to be the last.
Instead she became the living embodiment of Britain's national identity, and the country's longest reigning monarch.
The changes she made after that warning in 1957 were small but radical.
She ditched the annual balls held at Buckingham Palace for young aristocrats to make their formal debut into society. Instead she held garden parties for people from all walks of life.
She also started delivering her annual Christmas address on television, speaking to millions of her subjects from the comfort of her Sandringham living room.
"It's inevitable that I should seem a rather remote figure to many of you ... who never really touches your personal lives," she admitted in her first address.
"But now, at least for a few minutes, I welcome you to the peace of my own home."
Over dozens of Christmases, the Queen slowly won the affection, loyalty and reverence of her subjects.
Now the world is about to learn whether the woman who held the throne longer than anyone else leaves behind a Commonwealth made up of monarchists or Elizabethans.
Like his mother before him, Charles takes the crown from a cherished ruler.
And just like his mother, he must keep the United Kingdom, the Commonwealth and his own family together at a time when some are questioning whether such institutions are needed at all.
'Elizabeth the Silent': The Queen's mystique
To preserve the British monarchy "we must not let in daylight upon magic", British essayist Walter Bagehot famously warned in 1867.
He urged Queen Victoria to always remain above the fray.
"We must not bring the Queen into the combat of politics, or she will cease to be reverenced by all combatants; she will become one combatant among many," he wrote.
Queen Victoria's great-great granddaughter also embraced this advice.
She was the subject of countless documentaries, movies and television series, her image emblazoned on stamps and tea cups and the coins jangling in the pockets of millions of people.
And yet, despite a lifetime in the public eye, Queen Elizabeth remained shrouded in mystery.
"What she had to do was to maintain this strange thing that you have to [do] to keep the monarchy alive, and that's the mystique of the monarch herself, a kind of unknowability," Clive Irving, the author of The Last Queen: Elizabeth II's Seventy-Year Battle to Save the House of Windsor, said.
"Here you have this paradox: One of the most famous people in the world, yet you still know so little about her, and that's deliberate."
When Australia's referendum on becoming a republic failed in 1999, the Queen responded with polite neutrality.
"My family and I would, of course, have retained our deep affection for Australia and Australians everywhere, whatever the outcome," she said.
Even when the future of the 300-year old United Kingdom was in doubt during the 2014 referendum on Scottish independence, she said she simply hoped "people will think very carefully about the future."
Over the course of seven decades, the mask occasionally slipped. She appears to have backed sanctions against South Africa's apartheid regime in 1986, and was overheard in 2016 describing visiting Chinese officials as "very rude".
"In private she's sharp, she's funny, occasionally malicious, a good mimic," British historian David Starkey said on BBC Radio 4 in 2015.
But he said a "large padlock is placed upon the royal lips", earning the Queen the title of "Elizabeth the Silent".
Should Charles mimic the Queen's neutrality?
Elizabeth withstood family scandals, wars, economic depression, the loss of an empire, more than a dozen prime ministers, a pandemic and Brexit during her reign.
With a steady hand on the tiller through roiling social change, the Queen solidified her status as the most popular member of the royal family.
She had an 85 per cent approval rating in Britain in 2021, according to a YouGov poll. Two-thirds of respondents also said they would support the Queen if she decided to stay on the throne until her death.
Her popularity made her untouchable, even to those who opposed the monarchy.
When Malcolm Turnbull, the former head of Australia's republican movement, faced awkward questions as prime minister about leading a constitutional monarchy, he said his desire for an independent Australia could wait until after the Elizabethan era.
"The Queen has embodied selfless public service, dignity, wisdom, leadership far more magnificently than anyone alive today, there is no doubt," he said before they were due to meet in 2017.
And when Prince Harry and his wife Meghan sensationally claimed they were the victims of racism and cruelty from their own family, they were at pains to exclude the Queen from their criticism.
In contrast, Charles is a far less mysterious figure than his mother.
The details about the collapse of his marriage to Diana have been exposed. His most intimate conversations with Camilla were leaked. He is outspoken about his beliefs on climate change and conservation.
"Charles has a serious problem," according to biographer Clive Irving.
"I think we know far more than we would ever really want to know about Charles."
While Charles has vowed to rein in his outspokenness as sovereign, he has also refused to express any regrets for his activism.
"If it's meddling to worry about the inner cities as I did 40 years ago, if that's meddling, I'm very proud of it," he told the BBC in 2018.
Ultimately, biographer Catherine Mayer believes Charles will chart a very different course to his mother.
"Charles will never be neutral," she wrote in Charles: The Heart of a King.
"For better or for worse — in my final analysis, more often for better than for worse — the Prince is a man with a mission, a knight on a quest."
Can King Charles save the crown?
For 1,000 years, kings and queens have ascended the throne at times of great chaos. They have inherited wars and plagues, and rival claims to power.
But it could be King Charles who wears the crown during the monarchy's most treacherous era.
At a time when the world is reexamining traditional power structures, why should one man inherit power by virtue of his birth?
How can a white family accused of racism against its sole diverse member represent the hopes and dreams of 54 nations?
Why, at a time of such austerity and uncertainty, should the British taxpayer help fund the lifestyles of people already breathtakingly wealthy from a portfolio of land, property and assets?
They are questions Charles will have to answer, while also trying to keep a restless Scotland part of the union.
If the Scots break away, Wales might follow.
Australia's republican movement always saw the end of the Elizabethan era as its best shot at another referendum.
Polls show an increasing number of Canadians are also ready to stand on their own.
"I think there's a really real risk that if Charles does succeed [the Queen] that the monarchy will go over a cliff very fast," Clive Irving concludes.
"If the head of state turns out to be really wrong, things go down the tube very rapidly."
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