Ashen-faced, Prince Philip's close friend, Countess Mountbatten of Burma, looked 'heartbroken' as she arrived for the funeral yesterday.
Penelope Knatchbull – known as 'Penny' – sat quietly at the back of St George's Chapel.
In many ways, Penny, who turned 68 on Friday, was the second-most important woman in the Duke of Edinburgh's life – a constant confidante, loyal companion and 'keeper of secrets'.
'Penny was one of the few friends that Philip continued to see regularly after 2017 and his withdrawal from Royal duties,' said one aide.
'They were brought together by tragedy but were there for each other through thick and thin. He trusted her implicitly and she adored him. She never betrayed him.
'She was a keeper of not only his secrets but those of the entire family.
'Theirs was a deep and lasting friendship. Penny was always jokingly referred to as 'and also' because whenever a list was being drawn up for a family event, be it private or public, it would be 'let's invite X and Y' and then Philip would insist 'and also Penny…'
'No Royal event, major or minor, was complete without her.'
Penelope Knatchbull arrives at Windsor Castle to attend the funeral of Prince Philip
While much has been made of their shared passion for the sport of carriage driving, their ties were deep and went back decades.
Penny is thought to have met Philip in 1974 when she was dating Norton Knatchbull, now the 3rd Earl Mountbatten of Burma, one of Prince Charles's closest friends at Gordonstoun School in Scotland.
The only daughter of Reginald Eastwood, a butcher-turned-businessman, she was educated in Switzerland before taking a business degree at the London School of Economics.
Penny met her husband at a party thrown by mutual friends in London and was described by one acquaintance as 'one of the most natural young women I have ever met, outgoing but not brash or flirty. Utterly delightful'.
They added: 'She was also very bright, engaged and clever. Well read. She's easy company and lovely to be around'.
Pictured: Prince Philip and the Countess Mountbatten of Burma watch polo in 1975
Earl Mountbatten, 73, whose family seat is Broadlands in Hampshire, where the Queen and Duke of Edinburgh spent their honeymoon, is the grandson of Lord Louis Mountbatten, Prince Philip's beloved uncle.
When he married Penny in 1979 – with Prince Charles as best man – it was just two months after Lord Mountbatten had been murdered by the IRA. The Earl also lost his brother Nicholas and grandmother, Lady Brabourne, when a bomb, planted on the family's fishing boat, exploded in County Sligo.
Until 2005, the Countess was known as Lady Romsey and later Lady Brabourne until assuming her current title on the death of her husband's mother in 2017.
The Knatchbulls were always close members of the Royal circle but tragedy bonded Penny and Philip when, in 1991, her youngest daughter Leonora died of kidney cancer at the age of five.
Prince Philip, The Duke of Edinburgh, and Penny watch the driven dressage from the Prince's Landrover during day three of the Royal Windsor Horse Show in 2009
'He was a tremendous support for her during a time of unimaginable grief,' a source said.
'Despite their 30-year age difference, they had a lot of things in common. Penny has always been utterly discreet, totally loyal and always gave her honest opinion on things, good and bad, as best pals are supposed to do.
'But the friendship was a two-way street. Philip was always there for her.
Protocol puzzle over invitation
Mystery surrounded the inclusion of what was described as an 'odd' mention of Countess Mountbatten of Burma in the official 'commentary notes' released by Buckingham Palace to the media.
In them, the Countess – widely accepted as one of Prince Philip's longest and most loyal friends – was described as attending on behalf of her husband, rather than in her own right.
They declared: 'The Earl is unwell and unable to attend so The Countess is attending as his representative.
The Earl is the grandson of Admiral of the Fleet Louis Francis Albert Victor Nicholas Mountbatten, 1st Earl Mountbatten of Burma (born Prince Louis of Battenberg; 25 June 1900 – 27 August 1979), a British Royal Navy officer and statesman and uncle of The Prince Philip, Duke of Edinburgh.'
But one Royal observer described the reference as 'baffling', adding: 'It seems odd to put that paragraph in there and, frankly, slightly demeaning to Penny as she was far closer to the Duke than her husband ever was.'
'She looked heartbroken when she arrived at the funeral and she is heartbroken. She's lost her best friend.'
After Leonora's death, Philip began to invite Penny on carriage rides and she became passionate about carriage driving, a sport one aide described as 'one of the big loves of his life after the Queen'.
The Duke and Penny were often together at events such as the Royal Windsor Horse Show, sometimes on matching mini-motorbikes as they rode around the course they would later follow in their carriages.
Indeed, two black Fell ponies and a carriage designed by Philip stood at Windsor Castle yesterday as his coffin made its way to St George's Chapel.
The dark green four-wheeled aluminium and steel carriage, accompanied by two of his grooms, is one he designed and began using at the age of 91.
With it in Windsor Castle's Quadrangle were his two beloved Fell ponies – Balmoral Nevis and Notlaw Storm – both born in 2008.
In 2010, Earl Mountbatten left Penny to live with his mistress in the Bahamas, but within a couple of years had returned to a cottage on the Broadlands estate.
Penny took over the running of the 18th Century, 60-room mansion during her husband's absence and continues to run the house and estate today.
She allowed her errant husband to move back into the 'big house' after he was diagnosed with Alzheimer's. The Earl has not been seen in public for several years.
Their daughter Leonora is buried within the 86-acre grounds of Broadlands and Penny has devoted her life to raising money for a charity in her name.
Seven years ago, the Leonora Children's Charity Cancer Fund was merged with The Edwina Mountbatten Trust, a charity founded by Leonora's great- grandmother.
'Her life of service and charity is something which bonded her to the Duke,' the source said.
'There was a great mutual admiration between them.'
While many will miss Philip, the pain etched so clearly on Penny's face yesterday suggests few people outside his immediate family will miss his company more than her.
Prince Philip's closest confidante: Penny Brabourne who shared his love of carriage driving and was a regular visitor to Sandringham in his retirement
By Bridie Pearson-Jones for MailOnline
Penny Romsey, 68, who is 32 years younger than Prince Philip, remained close to the Duke of Edinburgh for years after he took it upon himself in 1994 to teach her carriage driving.
Formerly Penelope Meredith Eastwood, 'Penny' Knatchbull, previously known as Lady Romsey and later Lady Brabourne, is the daughter of a retired army major.
Penny's father left school at 15 and became a butcher, like his father and grandfather before him. He founded the Angus Steakhouse chain of restaurants which he sold for several millions, giving Penny a privileged childhood. She grew up and was educated in Switzerland before attending the London School of Economics.
She first met the Duke – who is 30 years her senior – at a polo match when she was 20 and in a relationship with Lord Romsey, Earl Mountbatten's grandson Norton Knatchbull.
She first met the Duke – who is 30 years her senior – at a polo match when she was 20 and in a relationship with Lord Romsey, Earl Mountbatten's grandson Norton Knatchbull (pictured the trio together in 2009)
Norton, 73, is the grandson of Lord Mountbatten - who was famously close to his nephew Prince Philip. Philip was Norton's godson, while Norton is the godfather of Prince William.
Penny's father, Reg Eastwood, had sold his steakhouse chain to the Golden Egg company and was living with his wife in Majorca when his daughter married Norton.
The wedding had been delayed for eight weeks because five months earlier, on August 25, IRA bombers blew up a small boat in the sea off Mullaghmore, Co. Sligo, where Lord Mountbatten had a holiday home.
It killed Mountbatten, Norton's 14-year-old younger brother Nicholas (after whom he was to name his own son), his paternal grandmother the Dowager Lady Brabourne and Paul Maxwell, a 15-year-old local.
Prince Philip with his confidante Lady Brabourne (Penny Romsey) at The Royal Windsor Horse Show in 2007 in Berkshire
Prince Philip and Lady Penny Romsey riding an 'Easy Rider' Monkey bike at Windsor Horse show in 2005 (pictured left) and in 2009, pictured right
Mountbatten's murder meant that Broadlands became the newlyweds' first and only home. Brought up in his parents' comfortable 18th century country house in Kent, Norton dreaded it. He never wanted the burden of Broadlands and knew he could hardly live up to his illustrious grandfather as the local 'lord of the manor'.
A family friend previously revealed: 'On the other hand, Penny was always comfortable there because she knew it was their duty.'
But Norton fell out with the locals when, in the Eighties, he tried to get planning permission for Tesco to build a superstore on the estate.
Feelings ran so high that opponents of the development carried a burning effigy of their High Steward through the streets of Romsey. The supermarket was never built.
The Queen and Duke of Edinburgh attended Penny's wedding to her husband Norton at Romsey Abbey in Hampshire
Royal expert Ingrid Seward previously said Prince Philip supported Penny when her husband Norton left her in 2010
Meanwhile, the family's original closeness to those in The Firm came through Norton's friendship with Prince Charles. This went back to when they were schoolboys together at Gordonstoun and Norton, a year older, was asked to show Charles the ropes.
In 1981, Penny and Norton welcomed their first child Nicholas Louis Charles Norton Knatchbull and a daughter Lady Alexandra a year later. In 1986, Penny gave birth to another daughter, who had kidney cancer and died aged five in 1991.
Just like his father and Charles, Nicholas was a year older than Prince William and was given the responsibility of showing him the ropes at Eton. In 2010, Norton moved to the Bahamas to embark on a new life with Lady Nuttall, 60. However, their affair fizzled out and he returned in 2014 to Broadlands estate in Hampshire.
Meanwhile, Prince Charles is also a close friend of the family, giving away Penny's daughter Alexandra at her 2018 wedding
Royal expert Ingrid Seward previously said Prince Philip supported Penny when Norton left her.
One of her oldest family friends previously revealed: 'I often wonder how their mother, Penny, copes with all the tragedy she has suffered.
'But she's a strong character - much stronger than Norton. I think Penny gets it from her father. He was a man who always seemed to know where he was going.'
She has always been close to the royal family, as one friend who has known her since those early days previously recalled: 'She was one of the most natural young women I have ever met, outgoing but not brash or flirty.'