One night, at a dinner party, the woman sitting next to Andrew Parker Bowles turned to him and smiled.
"I'm really hurt, Andrew," she said teasingly. "I'm the only one of Camilla's friends you haven't made a pass at. What's wrong with me?"
Everyone who knew the Parker Bowles well was aware of Andrew's serial unfaithfulness to Camilla, but it was passed off as a bit of a joke. It was nothing of the kind, of course.
All too often, the women were indeed friends of his wife, and showed scant loyalty to her by succumbing to his charms.
Yet Camilla never seemed to blame them or even have any stormy showdowns with Andrew, reports Daily Mail.
There was never a tense atmosphere in the couple's home; no barbed comments or bitter exchanges. Andrew's affairs were just a fact of life and not something Camilla often spoke about.
It was only those closest to her who knew quite how standoffish and cold Andrew could be towards her - and how deeply, bitterly hurt she was by his infidelity.
She loved him - for reasons her own family could never entirely fathom - and longed to be truly loved in return. But she didn't feel she was.
There was always someone prettier, wittier, sexier, waiting to take her husband away from her.
It wasn't that Andrew Parker Bowles was reacting against the ties of marriage. He had been unfaithful even when they were just going out.
They had first met in March 1965 at Camilla Shand's 'coming out' party as a debutante - a cocktail do for 150 people at Searcys, a smart venue behind Harrods in Knightsbridge.
She was just 17 but remarkably self-assured.
A "deb's delight" par excellence, Andrew was then 25 and a rather beautiful officer in the Household Cavalry.
Not only was he charming, smooth-talking and debonair but, thanks to his Army training and riding, he was slim and fit.
Another attraction was that he had noble blood and connections with royalty going back generations.
His parents - particularly his father, Derek - were close friends of the Queen Mother; and in 1953, at the age of 13, Andrew had been a page boy at the Queen's coronation.
Now, most of the upper-class women in London seemed to be after him, some of them married -and he knew it, and reaped the benefits.
Camilla, hugely popular with boys from an early age, had caught his eye at her coming out party and he had certainly caught hers. But they didn't meet again until 1966, at a dance in Scotland. He went over to her and said simply: "Let's dance."
They danced, and she fell in love. It was the start of a long, torturous romance - torturous because Camilla became a puppet on a string.
Andrew was hugely fond of her and she was nominally his girlfriend, spending many weekends with him at his parents' house near Newbury in Berkshire.
But he couldn't resist other women. And what was particularly hurtful was that - even then - many of them were Camilla's friends.
Occasionally, she retaliated. One night she spotted Andrew's car parked outside the flat of one of her best friends, so she wrote a rude message in lipstick on the windscreen and let all the air out of his tyres.
But, curiously for such a strong and confident woman, Camilla largely put up with his behaviour - possibly because she has also always been determined and stubborn. Once she had made up her mind she wanted to marry Andrew, nothing was going to stop her.
Five years later, however, there had still been no proposal. It was at this point that a good friend of Camilla's decided that she needed to meet the young and shy Prince of Wales.
Lucia Santa Cruz, a glamorous Chilean historian, had got to know Charles three years before when he was an undergraduate at Trinity College, Cambridge.
Back then, she had been working as a research assistant to the Master of his college, Lord Butler.
It was Butler - a friend of Lucia's father, the Chilean ambassador -who had invited her to dinner at the Master's lodge to meet the Prince, thinking they might enjoy one another's company.
They did; they became lifelong friends - but never in a romantic way. Although Lucia has repeatedly been credited with being the Prince's first lover, this could not be farther from the truth.
She already had a serious boyfriend - Juan Luis Ossa Bulnes, now her husband, and Charles is godfather to their eldest son.
By 1971, Lucia's parents had returned to Chile and she was living on the first floor of Stack House, a block of flats in Ebury Street, Belgravia.
One floor below, Camilla was sharing a flat with her friend the Hon. Virginia Carington, whose father was the Conservative politician Lord Carrington.
Lucia and Camilla already knew one another socially - they moved in the same circles - but as neighbours they became good friends.
They were in and out of each other's flats every day, borrowing clothes, going to the same parties and dances - and at weekends Camilla would take her friend down to her parents' house in the East Sussex countryside.
With her own parents so far away, this meant a great deal to Lucia. The Shands were warm and welcoming, even inviting her to spend Christmas with them and leaving a pillowcase full of presents at the end of her bed.
Inevitably, Lucia knew all about her friend's troubles with Andrew, and felt she deserved better.
The introduction to Prince Charles was easy to arrange: one evening in 1971, when Lucia and the Prince had already arranged to go out together, she asked him to arrive early.
She had "just the girl" for him, she added, describing Camilla as having "enormous sympathy, warmth and natural character".
Charles had recently been in Japan, where he had bought a little box as a present for Lucia. But once he knew he would be meeting Camilla, he brought a present along for her, too.
As Lucia made the introductions, she joked: "Now, you two, be very careful, you've got genetic antecedents. Careful, CAREFUL!"
As Charles and Camilla were well aware, she was referring to Alice Keppel, Camilla's great-grandmother, who had been a long-term mistress of Edward VII - Charles's great-great grandfather.
Intriguingly, Camilla's grandmother - Alice's daughter Sonia - was more probably the daughter of the King than she was of Alice's husband. Which meant that Charles and Camilla might well be related.
In Lucia's first-floor flat that evening, there was an immediate attraction between Camilla and the Prince, and an instant rapport.
He loved the fact that she smiled with her eyes as well as her mouth, and laughed at the same silly things as he did.
He also liked that she was so natural and easy and friendly, not in any way overawed by him, not fawning or sycophantic. In short, he was very taken with her, and after that first meeting he began ringing her up.
Then they started seeing each other. But it was a busy time for him, and he was seldom at home.
At 22, Charles was in the midst of intensive military training. He'd just qualified as a jet pilot with the Royal Air Force, and won membership of the exclusive Ten Ton Club by flying at more than 1,000 mph.
He'd also scared himself half to death by jumping out of an aircraft at 12,000ft, only to find that his parachute had wrapped itself round his feet.
Fortunately, he'd had the presence of mind to disentangle himself in time to descend harmlessly into the sea at Studland Bay in Dorset.
After his stint as a pilot, he'd embarked on a career in the Royal Navy, which entailed joining the destroyer HMS Norfolk for nine months -though he spent some of that time doing shore-based training courses in Portsmouth.
In the event, it wasn't until the late autumn of 1972, when Andrew Parker Bowles was away with his regiment, that Charles was on dry land for long enough to hook up with Camilla. Soon, they were seeing one another whenever the opportunity presented itself.
This was quite often at Smith's Lawn, the Guards Polo Club in Windsor Great Park, where both Charles and Andrew played for a time in the same team.
Camilla had been a regular sight at polo matches for years, watching Andrew and his friends play -and the father of one of her friends was chairman of the club. So she could go and watch Charles play without arousing particular attention.
By then, Andrew was serving in Northern Ireland; and this posting was followed by one to Cyprus. So for the last months of 1972, Charles had Camilla to himself.
And the two of them enjoyed some very happy times together, quite often at Broadlands - the Hampshire family seat of Charles's beloved great-uncle, Lord Mountbatten.
How did the old earl react to their trysts? In one sense, he was only too happy to play host to the pair - but he also made it abundantly clear to Charles that this relationship could never go anywhere in the long term.
Camilla was not sufficiently aristocratic to be the Prince's wife, and she was not a virgin, which to Mountbatten was a prerequisite.
Charles, however, was falling ever more deeply in love. And although far too reticent to say anything to Camilla, he was beginning to feel that, despite Mountbatten's warnings, he might have found someone with whom he could share his life.
The only cloud on his horizon was that in the New Year, he was due to leave for the Caribbean in the frigate HMS Minerva, which would take him away for at least eight months.
Three weeks before Christmas, he duly joined his new ship - and invited both Mountbatten and Camilla for a tour of inspection and lunch.
The weekend before he sailed, he was again at Broadlands with her, but he said nothing to indicate the strength of his feelings about her.
This was perhaps just as well, because she may not have known how to respond.
She was hugely fond of Charles and flattered by his attention. They'd had a very good time together - but Camilla was still in love with Andrew, who'd now returned to Britain.
To her fury, the man she adored, while still nominally her boyfriend, was now seeing Princess Anne, Charles's feisty younger sister.
This wasn't out of character, of course: Andrew had never been known to date just one girl at a time.
This time, however, he seemed to be unusually smitten; and rumour had it that Anne was, too. During their time together, he was even invited by the Queen to join the family at Windsor Castle for Ascot week.
So there was certainly an element of tit-for-tat in Camilla's fling with Charles. Indeed, her principal motivation was to have some excitement and make Andrew jealous. She knew the affair with Charles would never go anywhere, could never go anywhere.
Might she have felt differently if Charles had told her that he loved her, that he couldn't live without her and that he'd find some way of marrying her?
It's impossible to know. However, not even those who know Camilla best are convinced that she would have said yes to him.
True, Andrew had never been very nice to her. During the seven years they'd been going out together, he'd never made her feel special - and his playing-around had hurt her to the core.
Even so, she was still stubbornly determined to marry him. The fact that every other woman in London fancied him only made him more attractive to her.
She adored him, and she had been dating him through thick and thin for seven years. She wanted to be Mrs Parker Bowles, wife of her handsome cavalry officer, not Princess of Wales, not Queen.
As for Andrew, he was fully aware that his relationship with Princess Anne could never end in marriage.
However much in love they may have been, he was a Roman Catholic - and the 1701 Act of Succession expressly forbade any potential heir to the throne to marry a Roman Catholic.
For her part, Princess Anne, at that time fourth in line, wasn't about to cause a constitutional crisis.
Eventually, she turned her attentions to the man she later married - Mark Phillips, a captain in the Queen's Dragoon Guards, and a three-day eventer who'd just won a gold medal at the Montreal Olympics in 1972.
For a moment, Andrew - who knew about Charles - must have thought he was about to lose both his girlfriends.
So in March 1973, while the Prince was thousands of miles away in the West Indies, Andrew, now aged 33, finally asked Camilla to marry him. She accepted.
Then she wrote to Charles herself to tell him. Her letter broke the Prince's heart. In great distress, he fired off anguished letters of his own to his nearest and dearest.
It seemed to him particularly cruel, he wrote in one letter, that after "such a blissful, peaceful and mutually happy relationship", fate had decreed that it should last a mere six months.
He now had "no one" to go back to in England. "I suppose the feeling of emptiness will pass eventually," he concluded mournfully.
He did have one last-ditch attempt to get Camilla to change her mind, however. At the end of June 1973 - the week before the wedding - he wrote asking her not to marry Andrew.
Nevertheless, the wedding went ahead. Her mother Rosalind was not entirely happy about it - she thought Andrew didn't treat her daughter very well - but Camilla was determined.
She foolishly believed that leopards can change their spots and her heart belonged to Andrew, the man so many women had wanted but whom she had successfully bagged.
He thought he was everything she looked for in a man and he would give her everything she had dreamed of.
He was an alpha male, sophisticated and experienced. She liked the fact that he was a cavalry officer, as her father had been, and that like her father, who had won the Military Cross twice, he was brave.
He hadn't fought Rommel's tanks like her father, but in 1969 she'd watched him ride in the 129th Grand National, on a horse called The Fossa.
It is one of the most dangerous and challenging races over jumps in the world, and out of a field of 30 that year, only 14 finished. He was 11th.
By comparison, Charles at 24 was still a work in progress and would never match Andrew's confidence or his masculinity. It is no surprise that at the time she found him the more appealing.
The wedding went ahead on July 4, 1973, shortly before her 26th birthday. She looked glorious in a traditional white dress, with a 10ft train, by Bellville Sassoon.
Her bridesmaids wore mini-versions of her gown, while the page boys were in 19th-century military uniforms.
The guest list included some of the most illustrious names in the country, among them the Queen Mother, Princess Margaret, and Andrew's ex, Princess Anne.
Charles was also invited but didn't come, claiming another engagement.
Those close to him assumed that he stayed away for another reason: he simply couldn't bear to watch the woman he loved walk down the aisle with someone else.
After seven years of courtship, Camilla knew Andrew's whole family well. She was particularly fond of his father, Derek, a former soldier who was now the High Sheriff of Berkshire. But his mother, Dame Ann, was a difficult woman who displayed no great love for her eldest son.
Neither Camilla nor her brother-in-law Nic Paravicini - the husband of Andrew's sister Mary Ann - could cope with her, and they started a running joke about which of them was most in favour.
Camilla would say: "You're leading at the moment, I'm right at the bottom." But the next time they saw each other, their status would be reversed. She'd laugh her deep laugh and pull faces; laughter is her way of coping with every difficulty.
In 1974, when she was pregnant with her first child, the newly-weds bought Grade II-listed Bolehyde Manor, an imposing medieval property in the village of Allington, Wiltshire.
It was less than two hours from the capital, so ideal for Andrew, who needed to stay in London during the week as his regiment was based there.
A big attraction for Camilla was that it was just inside Beaufort country, home of the oldest and biggest fox hunt in England.
She'd hunted with her father since she was a little girl - and if she wasn't curled up with a good book, she wanted to be on a horse.
The house had 200 acres of land, stables, a swimming pool, a tennis court, a 17th-century stone dovecot, outhouses and two stone summerhouses.
Camilla soon began giving great dinner parties - starting off rather grandly with a Portuguese couple doing the work.
But after a year, she and Andrew decided it was a waste of money and she took over the cooking instead. She was good at it - her roast chicken is legendary.
But she sticks to what she knows, which is mostly plain English fare with lots of fresh vegetables.
As well as a vegetable garden, Bolehyde had a resident ghost, but Camilla was typically unfazed.
She'd joke about how she'd be sitting on the sofa, watching television, when the ghost would come and sit beside her and change channels. She never saw it, but she could feel it next to her, she said.
The newly-weds soon settled down to routine of weekdays mostly apart, with Andrew in London, and weekends normally together.
Whenever he went back to London, he'd leave Camilla lists of chores to do in his absence.
She and her friends used to laugh about the lists, most of which she ignored. Camilla was not built for work in those days - she did the bare minimum she could get away with - but she was a good homemaker and an excellent mother.
The house was dark inside because of the leaded windows and low ceilings, but she brightened it with a mixture of antique and modern pieces, using pretty fabrics, plenty of table lamps and good rugs.
There were books everywhere, and dozens of prints, paintings, cartoons and photographs on the walls, while knick-knacks covered all available surfaces.
Definitely shabby-chic rather than - to use Camilla's expression - "tickety-boo", it was a comfortable and happy home for her two children, Tom (born in 1974) and Laura Rose (1978).
To most outsiders, the Parker Bowles seemed to be a fortunate and very happy family. But appearances were deceiving.
Because Andrew was spending his weekdays in London, he was never short of opportunities to be unfaithful.
Today, looking back, he would admit that if blame were to be apportioned for the way his marriage ended, he'd feel obliged to take a full 80 per cent of it. Love her though he does, he would also admit that Camilla was more in love with him than he was with her.
This was certainly evident from his behaviour in London, where he shared an apartment with his brother-in-law Nic Paravicini - the same flat Camilla had once shared with Virginia Carington. Although both men were married, they led a bachelor existence
They had a code involving empty milk bottles. Arranged in a certain way outside the door, these meant 'Do not disturb'. And Nic would readily agree that Andrew arranged the milk bottles more often than he did.
But for all the hurt this caused for Camilla - particularly when her husband dallied with her so-called friends - it never occurred to her to divorce him.
She'd been brought up to believe that you stuck at things, you didn't give up. And so she found ways of coping.
Hunting was one way; galloping among a cavalcade of horses left no time for rumination. And in the summer, when the hunting season came to an end, her escape was to work in the garden.
Every summer, too, her parents paid for the entire family to spend a fortnight in the Grand Hotel Excelsior on Ischia, a tiny volcanic island south-west of Naples.
But Andrew was never keen on the sun; he'd spend his time inside, observed one relative cattily, writing postcards to duchesses and all his other titled friends.
Camilla certainly never set out to be unfaithful to Andrew. She flirted for sure, because that was the way she was - a twinkly, sexy woman with a husky laugh that men adored, and a tendency to say outrageously funny things.
But she no more wanted an open marriage than to fly to the moon.
As the years went by, however, she came to realise that her husband would never change; he would never love her and cherish her, never make her feel good about herself. And inevitably, the confidence that had been her hallmark throughout her childhood began to crumble.
What had made her so strong as a child was the absolute certainty that her parents loved her, and the absolute security that came from that certainty. She never, ever had that feeling with Andrew.
She lived with a permanent knot of dread in her tummy, fearing that one day he might leave her. And this left her very vulnerable to the attentions of a suitor.
Charles had been heartbroken when he lost Camilla to Andrew; but however strong his feelings for her, life had to go on.
He was young, he was attractive, he was eminently eligible and he took out a succession of pretty girls, some of them suitable, some not - and some too sensible to want a future under the spotlight.
Every woman he dated risked having her past raked over in the papers. Now approaching 30, he despaired of ever finding a bride.
Meanwhile, although his affair with Camilla had ended before her marriage, it was inevitable that they'd continue to see one another. They had many friends in common, and went to many of the same sporting and social gatherings.
Charles and Andrew were friends, and still played in the same polo team. And since a lot of the matches were held in the Wiltshire/Gloucestershire area, it was logical for the Prince to stay the night at Bolehyde.
Then, because of Andrew's royal links, there were often invitations from the Queen and the Queen Mother as well as from the Prince himself.
So the Parker Bowles went to stay at Sandringham and Balmoral; they went racing at Ascot and Cheltenham; and they were always invited to the lavish parties the Queen Mother threw at Royal Lodge in Windsor Great Park or at her Scottish homes, Birkhall and the Castle of Mey.
Andrew was a great favourite of the Queen Mother, and she approved of his new wife. So did the Queen; their mutual love of dogs and horses was a winning formula, and she got on famously with Camilla.
When Tom was born, the Parker Bowles asked the Prince to be their son's godfather, which further cemented the relationship between them all. They even arranged for the christening date to fit in with Charles's naval schedule.
He finally left the Navy in 1977 - the year of the Queen's Silver Jubilee, and the beginning of his full-time royal duties. This was also the year he dated Diana's eldest sister, the fiery, red-haired Lady Sarah Spencer.
They'd met at a party at Windsor Castle, to which Andrew and Camilla - then pregnant with Laura - also came.
But the relationship with Sarah came to an abrupt end when she told the Press that Charles fell in love easily, but she wasn't in love with him.
Such publicly bruising comments did nothing for his fragile ego. And where better to seek solace for life's knocks, setbacks and dilemmas than with Camilla - who might laugh and tease him but would do so gently and kindly, almost in a maternal way, and who was loyal to her bootstraps.
By the late Seventies, she'd become quite simply his best friend. He drew from her strength and felt safe and comfortable around her.
And he loved that he could talk to her openly and honestly about everything, knowing that nothing would go any further.
Their friendship was purely platonic. And he visited often, enjoying being in a normal home, and just being around her and her small son, his godson.
Friends of Camilla's would arrive at Bolehyde to find Charles sitting on the floor, watching television with Tom - and do a double-take when they realised who it was.
By now Charles had joined the Beaufort Hunt, so they also had a love of hunting in common.
They shared many other enthusiasms, and when he was away from Camilla, Charles would speak to her on the phone for hours, pouring his heart out, or he'd send her long expansive letters - all of them handwritten with a fountain pen in black ink.
His head was always buzzing with ideas and torturous thoughts about life and his curiously ill-defined role in it.
Conversation came easily between them and they shared the same sense of the ridiculous.
She didn't falsely flatter him, she wasn't angling for anything, and she was one of the few people who'd never been in awe of his status.
Just as she always had, she treated him like a normal person - and if ever he behaved badly, or was selfish or thoughtless, she wasn't afraid to tell him so.
As time went by, the nature of her relationship with Charles changed. In either 1978 or early 1979, after the birth of Laura, they became lovers again.
This had not been in Camilla's game plan when she married Andrew.
She was not a natural adulteress and there had been no other men in her life apart from the Prince of Wales in all the years she had been with Andrew, despite his flagrant adultery; but she was tired of sitting at home in the country while he played around with other women. It was high time he knew how it felt.
Andrew was in no position to complain; and when he discovered what was going on, he wisely didn't make a fuss.
Some would say that a part of him actually quite enjoyed the fact that his wife was sleeping with the future King; he might have felt differently had Charles been a travelling salesman.
Whatever the truth of that, Andrew wasn't overly worried about Camilla's affair.
After all, everyone close to the Prince knew that he was already sleeping with Dale Tryon - a socially ambitious Australian - and one or two others.
And, in any case, Andrew wasn't the jealous type. Plus he assumed that there was never any danger that the affair might bring an end to their marriage.
He knew Camilla was flattered by the Prince's adoration, but he didn't think she was in love with him - and he knew perfectly well that Charles could never marry a divorcee.
So life went on much as it had before. The Prince still came to stay with the couple: they all went racing together, to polo, to parties and dinners and balls.
They all behaved in a typically upper-class fashion, as if nothing had happened. There were no rows, nothing that upset the children or gave them any cause for concern.
The children were Camilla's priority and she was very discreet, but it wasn't long before other people had their suspicions.
The Prince is cursed by the need to have personal protection officers (PPOs) with him at all times, and he also has a team of people to look after him, plan logistics and organise his diary, even his social diary.
He can do nothing without someone knowing where he is and who he's seeing, and the royal court is a hotbed of gossip, as it always has been, and always will be.
Some of those closest to him began to voice their suspicions that Camilla had been added to the list of women who were more than just friends.
Members of his own family picked up on it and warned that if news got out, it could be very damaging, not just to him as Prince of Wales but also to the institution.
The days had long gone when princes could flagrantly exercise a royal droit de seigneur and sleep with other men's wives, as Edward VII had done, and the once compliant Press could no longer be relied upon to protect the Crown from scandal.
But by then, Charles was far too dependent on Camilla to be warned off by the risk of discovery.
Adapted from The Duchess: The Untold Story by Penny Junor, published by William Collins on June 29 at £20. © Penny Junor 2017. To order a copy for £15 (offer valid to July 1, 2017, p&p free), call 0844 571 0640 or visit www.mailbookshop.co.uk.