The loss of her beloved husband, the man she called the rock, will be one of the most traumatic things the Queen has ever had to face.
Together for more than 70 years, until his death on Friday morning, the Duke of Edinburgh lived through the ups and down of the monarch’s life and reign with her.
In 2002, her Golden Jubilee year, the Queen lost both her mother and sister within weeks of one another. But Philip was at her side, as always.
There will be no chance to spend as much time as she needs to grieve out of the spotlight.
An appearance at the duke’s funeral in the full glare of the media beckons 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 then prime minister Winston Churchill and other dignitaries immediately on returning to London from Kenya 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.
During her Diamond Jubilee celebrations, she cut a solitary figure as she walked through St Paul’s Cathedral without the duke, who was in hospital with a bladder infection.
Princess Eugenie, the couple’s granddaughter, said 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 jubilee events.
“They are the most incredibly supportive couple to each other.
“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,” she told Sky News.
The couple reached the rare personal milestone of their platinum wedding anniversary on November 20 2017, 70 years of marriage, and, in a touching gesture, the Queen appointed Philip a Knight Grand Cross of the Royal Victorian Order for his years of devotion and duty.
They 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 together saw the rapid advances in modern life from man walking on the moon for the first time to the invention of the internet.
As duty dictates, the Queen will continue her role as sovereign without the life-long companion upon whom she greatly depended.
Despite her role as Queen, Prince Philip never shied away from telling his wife things as they were.
Lord Charteris, the monarch’s former private secretary, once recalled: “Prince Philip is the only man in the world who treats the Queen simply as another human being.
“He’s the only man who can. Strange as it may seem, I believe she values that,” he told royal writer Gyles Brandreth.
The success of the couple’s long-lasting marriage, which spanned more than 70 years, was put down to their compatibility. They shared interests and had the same dutiful royal training.
Both loved horses and the outdoor life; both were undemonstrative by nature, regarding displays of emotion as something to be kept private.
Yet in character, the Queen and Philip were markedly different.
She was passive, cautious and conventional; he was more adventurous, tempestuous and active.
Their partnership was a traditional one and the Queen grew up in a world where it was the man who was in charge behind closed doors.
A friend of the royals once told biographer Sarah Bradford: “He shouts at the Queen sometimes like he shouts at other people and she doesn’t seem to mind.
“It’s as if she thinks that’s how husbands behave.”
There were moments when Philip’s bluntness would embarrass the Queen, but she was not averse to telling him herself to “Shut up”.
She knew how to handle him and would retort: “Oh Philip, do shut up. You don’t know what you’re talking about.”
When he shouted at her in private, she would calmly ignore it.
The Queen clearly adored the duke. During public engagements, she could sometimes be seen waiting for him to catch up with her as he lingered behind entertaining the guests.
“Where’s Philip?” she would ask and, on catching sight of him walking towards her, she would give a small chuckle and flash him a beaming smile.
In the early days of their marriage, Philip was sometimes strident and, according to close friends, Elizabeth would not fight back.
Later she learned to resist and, in the end, the royal couple achieved a kind of harmony, said the Queen’s biographer, Ben Pimlott.
But their domestic relationship could perhaps best be illustrated by an anecdote recounted by Lord Mountbatten.
Philip, who had a record of minor motoring accidents, was driving his wife and Uncle Dickie to Cowdray Park.
The Queen, worried about the speed at which Philip was going, started to tense herself and draw in breath.
Eventually her husband turned to her and barked: “If you do that once more I shall put you out of the car.”
She stopped immediately.
When they arrived, Lord Mountbatten asked her: “Why didn’t you protest? You were quite right – he was going much too fast.”
The Queen looked puzzled and replied: “But you heard what he said.”
The difficulties faced by the royal family during 1992, the Queen’s “annus horribilis”, were said to have brought the couple closer together.
The duke, who had mellowed with age, became noticeably more considerate and supportive.
“He’s unsung for the total support he gives to the Queen,” a Royal Household insider once told author Sarah Bradford. “They’re like Darby and Joan now – it’s very sweet really.”
While on honeymoon Princess Elizabeth wrote to the Queen Mother, in letters seen by biographer William Shawcross, and declared: “Philip is an angel – he is so kind and thoughtful and living with him and having him around all the time is just perfect.”
Philip in turn wrote to tell his mother-in-law of his deep love for his new wife.
“Lilibet is the only ‘thing’ in the world which is absolutely real to me and my ambition is to weld the two of us into a new combined existence that will not only be able to withstand the shocks directed at us but will have a positive existence for the good,” he said.
On the Queen’s accession, Philip watched her become the single most important woman in the country and discovered that his own children would not be allowed to take his name.
He later won a part-concession when in 1960 it was announced that the Queen’s descendants, when they needed a surname, would use Mountbatten-Windsor.
He devoted his married life to supporting his wife, giving up his naval career to be by her side.
At the start of her reign, he helped her in particular with her public speaking, encouraging her to lower her voice when she spoke.
He was incredibly protective of her and would berate photographers who got too close.
A cruise of Commonwealth countries in 1956 took Philip away from the Queen for four months, prompting rumours of a rift between them.
But there was no evidence that all was not well.
On their golden wedding anniversary in 1997, the duke praised his wife for her abundance of tolerance.
“I think the main lesson that we have learnt is that tolerance is the one essential ingredient of any happy marriage,” he said.
He added: “It may not be quite so important when things are going well, but it is absolutely vital when the going gets difficult.
“You can take it from me that the Queen has the quality of tolerance in abundance.”
The Queen spoke of him fondly in return.
“All too often, I fear, Prince Philip has had to listen to me speaking,” she said.
“Frequently we have discussed my intended speech beforehand and, as you will imagine, his views have been expressed in a forthright manner.
“He is someone who doesn’t take easily to compliments but he has, quite simply, been my strength and stay all these years, and I, and his whole family, and this and many other countries, owe him a debt greater than he would ever claim, or we shall ever know.”
In 2007, the couple celebrated their diamond wedding anniversary by travelling to Malta, where they had lived for a short time as a young couple.
Five years later, they marked their blue sapphire anniversary – 65 years – and in 2017 passed the rare, personal milestone of 70 years of marriage – their platinum wedding anniversary.
