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Watch: Prince Philip 'would have urged Queen Elizabeth to relax a bit'
At 95, the Queen is three decades older than the average retiree – yetshe is still working relentlessly, from meetings with the prime minister to a packed schedule of visits and events.
Last week an unexpected hospital stay had aides flustered, and they initially concealed the fact that she had remained in hospital overnight.
She was said to be "resting" and "in good spirits" but cancelled a planned visit to Northern Ireland at short notice, and missed her usual attendance at church in Windsor on Sunday.
It's now unclear whether she will appear as planned at the COP26 climate conference in Glasgow, which begins on 1 November.
Recently she has been seen using a walking stick "for comfort", and after being widowed last spring after a 73 year marriage to Prince Philip, is living at Windsor Castle, reliant on staff and visitors for company.
"Following medical advice to rest for a few days, The Queen attended hospital on Wednesday afternoon for some preliminary investigations, returning to Windsor Castle at lunchtime today, and remains in good spirits," read the official Palace statement.
It's clear that, much as many wish the Queen would carry on indefinitely, the time for her to dial back her commitments and ease into a less hectic lifestyle must be imminent.
It has now been suggested that Her Majesty will be joined by another family member on royal engagements in future, in case she has to cancel at short notice.
Aides are also compiling a list of "key events" for her, mostly related to next year's Platinum Jubilee celebrations.
But who will step up and be available to attend events with The Queen? The royals all have busy schedules of their own, and are regularly out of London at their country homes – Highgrove in Gloucestershire for Charles and Camilla, Anmer Hall in Norfolk for William and Kate and Gatcombe Park in Gloucestershire for Princess Anne.
Prince Edward and wife Sophie live close to Windsor Castle at Bagshot Park in Surrey, part of Windsor Great Park.
Of course, the Queen's middle son Prince Andrew is not an option – some time after his links to paedophile sex trafficker Jeffrey Epstein were revealed, Andrew stepped down from public life.
And while in the past, Prince Harry was often seen at his grandmother's side, his decision to rescind royal duties and move with wife Meghan to California means he has not seen much of his family since – and will certainly not be available to support the Queen over the coming years.
Who, then, will be the royal consort, available to accompany the Queen at short notice and take on any engagements she's forced to forego? Here are the likely contenders.
Prince Charles and Camilla
Charles is first in line to the throne, so it would make perfect sense for him to accompany his mother to events and meetings.
He will already have access to the top echelons of royal-eyes-only information, and has spend decades waiting for his accession.
But in April, a poll by The Mirror newspaper found that the public would prefer the crown to go straight to William, bypassing Charles – 47% backed the crown skipping a generation and just 27% wanted the crown to go to Charles.
“I imagine he’ll be feeling very disappointed and depressed," said royal biographer Penny Junor of the findings.
“I’m sure he’s really proud of William. I’m sure he’s really proud of the incredible work he has done and the success he has made of his marriage and family. But he also doesn’t want to be erased just yet.”
It's likely that despite his lack of public popularity, Charles will want to take the chance to showcase his suitability for the role of king, and Camilla will accompany him and the Queen on occasion, putting her admired "people skills" to use.
However, this will mean the couple scaling back their own commitments, which include the patronages of various charities, in order to be on standby for the Queen.
Prince William and Kate
Hugely popular, Prince William is currently riding high on a wave of goodwill after the successful launch of the Earthshot Prize awarded to climate change projects.
His wife Kate is also much loved by the public, and in the wake of Harry and Meghan's departure, the nation has embraced the Cambridges and their three adorable children. There's no doubt either one of the couple would be a successful sidekick to the Queen.
However, both are hands-on working parents with big projects to oversee and promote, William's involving climate change and Kate's focused on early years care and education. They are both patrons of many charities and organisations, and regularly required at related events.
With all three children at home and still under 10 and a packed schedule, having already taken on extra work after Harry's departure, being available at a moment's notice for the Queen may prove impossible,
Edward and Sophie Wessex
In recent years, the Queen has reportedly grown close to Sophie Wessex, who is herself increasingly popular with the public, campaigning for better understanding of menopause in the workplace and opening up over her grief at Prince Philip's death.
Sophie is mother to Lady Louise, almost 18, currently at boarding school, and James, Viscount Severn, 13.
Living close to the Queen's permanent residence in recent years, Sophie is a frequent companion for her. According to royal correspondent Richard Kay, they also share a fascination with military history.
"The pair are sometimes gone for hours, pouring over ancient documents in the Royal Archives, which are kept at Windsor Castle.
Watch: Prince Philip's death has left 'giant-sized hole' in Royal Family, says Sophie Wessex
“When Sophie visited the World War One and World War Two battlefields and cemeteries in France, she told the Queen all about what she had seen and what she felt.”
A royal aide told the Daily Mail that Her Majesty "talks to Sophie in the way she used to talk to Princess Margaret. Sophie has filled a terrible gap in the Queen’s life that was left when her sister and the Queen Mother died in 2002.”
Another aide said: “She is like another daughter to Her Majesty, they are that close.”
With older children and close proximity to The Queen, Sophie and Edward are ideal contenders to take on the extra duties – although they have already stepped up significantly after Harry and Meghan's departure.
Perhaps the most dutiful of all the Queen's children, Princess Anne, 71, lives at Gatcombe Park in Stroud, just under two hours' drive from Windsor. Married to Commander Tim Laurence, she is known to have a hectic schedule of events and was the hardest-working royal last year, with 110 engagements in 106 working days.
In her role as the Princess Royal, Anne performs official duties and engagements on behalf of the Queen and is patron of over 300 organisations, and 200 charities focused on sport, sciences, people with disabilities, and health in developing countries.
In 2014, Kevin S MacLeod, the Canadian secretary to the Queen, said: "Her credo is, 'Keep me busy. I'm here to work. I'm here to do good things. I'm here to meet as many people as possible."
But dutiful though she is, her working life is already crammed full, and it's also known that she loves to spend time with her four grandchildren who all live on the Gatscombe Park estate.
With her strong sense of duty, and loyalty to the Queen, however, it's likely that Anne will be on call – and will go wherever she's needed.