The loss of her beloved husband at the age of 99 will be one of the most traumatic experiences the Queen has ever had to face.
Together for more than 73 years, the Duke of Edinburgh has supported the monarch through the ups and down of her life and reign, including throughout the time they have spent together at Windsor Castle as part of 'HMS Bubble'.
His death comes during one of the Royal Family's most testing periods, with Buckingham Palace plunged into crisis following Harry and Meghan's explosive interview with Oprah Winfrey.
As Philip languished in hospital, the CBS programme aired on March 7 and shook the monarchy to its core as the Sussexes made accusations of brazen racism within the royal ranks.
And the Queen had to cope with all this while her husband was spending 28 days in hospital in London as he was receiving treatment at both King Edward VII's Hospital and St Bartholomew's Hospital.
Both Philip and the Queen were cleared of being the culprit who expressed 'concerns' about 'how dark' Archie's skin would be - but the claim has serious implications for the institution.
The Queen wearing a tartan skirt with corgis beside her and Prince Philip wearing a kilt in Balmoral in 1994
The Queen and the Duke of Edinburgh laugh as they bid farewell to Irish President Michael D. Higgins and his wife Sabina at Windsor Castle after their state visit in April 2014
During her Diamond Jubilee celebrations in June 2012, the Queen cut a solitary figure at St Paul's Cathedral without the Duke, who was in hospital with a bladder inflection at the time
The Queen and Philip welcomed a new great-grandchild - their eighth - with the birth of the Duke and Duchess of Sussex's son, Archie Mountbatten-Windsor. Pictured left to right in June: Prince Philip, Prince Harry, Queen Elizabeth, Doria Ragland, and Meghan
The Queen and Philip, pictured at Broadlands in 2007, shared an irreplaceable bond - united at key moments of history, witnessed from the unique viewpoint of a monarch and her consort
The Queen and His Royal Highness The Duke pictured against a platinum-textured backdrop in recognition of their special anniversary in 2017. The Queen is wearing a cream day dress by Angela Kelly and a 'Scarab' brooch in yellow gold, carved ruby and diamond, designed by Andrew Grima, and given as a personal gift from the Duke to The Queen in 1966
The Queen and Prince Phillip enjoy the spectacle, as a swarm of bees cause concern prior to The Queens Company Review at Windsor Castle in April 2003
Philip and the Queen laugh at the Royal Highland Games at Braemar in September 2003, in front of Tony and Cherie Blair
Although Philip stepped down from doing public engagements in 2017, he had been by the Queen's side throughout the rollercoaster of the past year.
The monarch had to cope with Prince Harry's apparent feud with Prince William and criticism of The Duke and Duchess of Sussex for taking private jet journeys despite speaking out on environmental issues.
Most recently she had to deal with the upset caused by Megxit, which Harry and Meghan decided to move to an £11million mansion in California rather than continue living in Frogmore Cottage in Windsor.
Then in January 2020 when the Duke and Duchess of Sussex shocked the world by announcing their intention to step down as senior royals.
Buckingham Palace said all were 'saddened' by their decision to permanently step down as working royals, but they remained 'much loved members of the family'.
A statement added that the Queen had 'written confirming that in stepping away from the work of the Royal Family it is not possible to continue with the responsibilities and duties that come with a life of public service'.
But the Sussexes hit back with a statement of their own, saying: 'We can all live a life of service. Service is universal.'
Philip's son Prince Charles, 72, visited him at King Edward VII Hospital in London on Saturday afternoon for half an hour after making a 100-mile journey from Highgrove in Gloucestershire to the capital, and appeared emotional when he left.
Royal expert Penny Junor suggested that while she did not know the reason for Charles's visit, Harry is likely to have come up.
She said: 'Momentous things are happening in the family at the moment and I suppose it's perfectly possible that Charles wanted to go and talk to his father and reassure him about Harry.'
The Queen has, of course, previously experienced great loss. In 2002, her Golden Jubilee year, both her mother and sister died within weeks of one another. But Philip was at her side.
Princess Elizabeth photographed in Clarence House in July 1951, with the Duke of Edinburgh
Queen Elizabeth II, wearing the Imperial State Crown, and Prince Philip, in uniform of Admiral of the Fleet, wave from Buckingham Palace in London after the Coronation in June 1953
Queen Elizabeth II on the balcony at Buckingham Palace after her coronation, on June 2, 1953. With her are (left to right): Prince Charles, Princess Anne, Prince Philip, Duke of Edinburgh and Queen Elizabeth The Queen Mother
The Queen holds the Orb and Sceptre at her Coronation in June 1953, which took place at Westminster Abbey in London
Princess Elizabeth (now Queen Elizabeth II) at the Royal Opera House in Covent Garden for a gala performance in July 1951
There will be no chance to spend as much time as she needs out of the spotlight grieving, with an appearance at the Duke's funeral in the full glare of the media beckoning within days.
The Queen, with all her training as head of state, is used to holding her emotions in check in public.
Shortly after her father King George VI died, she was required to greet Prime Minister Winston Churchill and other dignitaries immediately on landing in London as the new Queen.
But sometimes the depth of her sadness has proved too much.
Just months after the Queen Mother died, tears rolled down her cheeks when she took on her late mother's role at the poignant opening of the Field of Remembrance at Westminster Abbey.
The Queen at a polo match with the Duke of Edinburgh in 1955
The then Princess Elizabeth and the Duke with their two young children, Princess Anne and Prince Charles, outside Balmoral Castle in Aberdeenshire on September 19, 1952
Princess Elizabeth and the Duke of Edinburgh, on honeymoon, photographed in the grounds of Broadlands looking at their wedding photographs, on November 23, 1947
During her Diamond Jubilee celebrations in 2012, she cut a solitary figure as she walked through St Paul's Cathedral without the Duke, who was taken to hospital with a bladder infection.
Princess Eugenie, the couple's granddaughter, described how the Queen and Philip were each other's 'rock' and spoke of how difficult it had been for the Queen to be without her husband during the Diamond Jubilee.
'They are the most incredibly supportive couple to each other,' she told Sky News after the Jubilee.
'Grandpa was unfortunately taken ill and for Granny to come and do that alone was probably quite testing and I think he is her rock, really, and she is his.'
Queen Elizabeth II with Prince Philip, Prince Charles and Princess Anne at Balmoral in September 1952
The infant Prince Charles is pictured in the lap of his mother, the then Princess Elizabeth, with his father Prince Philip in 1948
The Queen, Duke of Edinburgh, Prince Charles, Prince Andrew, Prince Edward and Princess Anne at Balmoral in August 1972
The Queen and the Duke shared an irreplaceable bond - united at key moments of history, witnessed from the unique viewpoint of a monarch and her consort.
While private secretaries and household staff have come and gone, Philip remained a constant in the Queen's life.
They travelled the globe together, endured state visit after state visit, and thousands of engagements over the years - all made more bearable with one another's company and through the knowledge they were in it as a duo - albeit one wearing the crown.
They also witnessed the changing world from a shared standpoint, with just five years difference in age between them.
They married in the 1940s and saw together the rapid advances in modern life from man walking on the moon for the first time to the invention of the internet.
The Duke of Edinburgh and Princess Anne preparing a barbecue on the Estate at Balmoral Castle in August 1972
Prince Philip and Prince Charles share a joke at a Guards Polo Club tea party in 1999
Now as duty dictates, the Queen will continue her role as Sovereign alone, without the lifelong companion upon whom she greatly depended.
The Queen has been gradually reducing the number of public engagements she attends from 332 in 2016 to a still impressive 283 in 2018.
Her son Prince Charles, meanwhile, undertook 507, and he met Prince Philip at Sandringham in November 2019 to discuss the fallout from Andrew's BBC interview.
Charles's key role in 'retiring' Prince Andrew from public life has fed speculation he is preparing to adopt a modern 'Prince Regent' role.
The Queen displays a brooch given to her by Philip in 1966. She is pictured wearing it in a 2017 portrait photograph
This would see him control day-to-day royal affairs while his mother remains monarch.
Her Majesty will turn 95 in April 2021 - the same age at which her husband Philip withdrew from his public duties.
And there is talk among courtiers that she may use the milestone to effectively hand over day-to-day control of the monarchy to Charles.
Many courtiers feel that since Philip, who used to rule his family with an iron fist, retired from public life, 'discipline' within the royal family has not been what it should be.
Princess Elizabeth, Britain's future queen, and Lieutenant Philip Mountbatten shown at Buckingham Palace following their engagement, in November 1947. On her engagement finger, Elizabeth wears a three-diamond ring which she wears to this day
The Queen's engagement ring is pictured as she attends the 2007 Windsor Horse Show
The Queen toasts Prince Philip at the opening of the Millennium Dome in London on New Year's Eve 1999
This has been linked to Andrew's virtually autonomous decision to go ahead with his disastrous Newsnight interview about Epstein.
Royal biographer Christopher Wilson described him as 'the guiding hand, the disciplinarian' and said that since his retirement there was no central command at Buckingham Palace, telling the Daily Telegraph: 'You can see the disintegration.'
Following this the Duke and Duchess of Sussex announced their departure from royal duties at the start of 2020, as they moved to North America for a new life.
The couple, who had their first son Archie in May 2019, decided to step down following a series of controversies including their support for environmental issues despite regularly flying on private jets.
'Megxit' was then made permanent in February 2021, after Buckingham Palace confirmed Harry and Meghan - who are now expecting a second child, a girl - would not be returning as senior royals.
But weeks later the the monarchy was plunged into crisis while Philip was in hospital following the shocking allegations of racism made by the Sussexes in their interview with chat show host Oprah Winfrey.
Harry and Meghan, who faced calls to postpone the interview because Philip was unwell, accused an unnamed royal of raising concerns about how dark Archie's skin tone would be before he was born.
In March this year, Philip was reunited with the Queen after leaving hospital following a period of 28 days receiving treatment at both King Edward VII's Hospital and St Bartholomew's Hospital in London
He was initially receiving care for an infection then underwent heart surgery for a pre-existing condition. Philip was taken to King Edward VII's by car on February 16 after feeling unwell at Windsor Castle.
Two weeks later was moved to St Bartholomew's Hospital in the City of London by ambulance where he had a successful procedure on a pre-existing heart condition on March 3.
A few days later he was transferred back to King Edward's to recuperate and to continue his treatment - before being taken back to Windsor Castle on March 16 after a month away from his wife.