Tragedy of the other Prince William: How the dashing but doomed young royal's affair put the Palace in a spin

Had history turned out differently, Zsuzsi Starkloff may well have been a member of the English aristocracy. Her title, Duchess of Gloucester, would bring with it an apartment in Kensington Palace and a sprawling estate in Northamptonshire.

Instead, the 79-year-old former model lives in a modest home in Colorado. Look closely though and there are hints of a more exotic past, in the photos scattered about and in the signet ring she wears on a chain around her neck - it bears the initial 'W' for William, the name of her former lover.

This was not just any William either. The ring belonged to Prince William of Gloucester, the Queen's cousin, a dashing young man who captured Zsuzsi's heart. Theirs was a passionate 60s love affair that caused panic in Establishment circles: Hungarian, twice divorced with a daughter - not to mention Jewish - Zsuzsi was not considered a suitable bride, certainly not for a prince who, when born, was fourth in line to the throne.

Prince William of Gloucester, the Queen's cousin, pictured in 1962: Like his uncle Edward VIII before him, who'd given up his throne to marry Wallis Simpson, William had to wrestle with the conflicting pull of duty and love

Like his uncle Edward VIII before him, who'd given up his throne to marry Wallis Simpson, William had to wrestle with the conflicting pull of duty and love, although ultimately it was a tragic twist of fate that decided the course of his life: in 1972 the prince died when his plane crashed during an air race. He was just 30. 'My heart was broken in that moment,' Zsuzsi says today.

Months away from her 80th birthday, Zsuzsi has never spoken about the affair on camera before. 

But now her story is told in a fascinating documentary which traces the tragically short life of a prince who was so admired by a young Prince Charles that, years later, he would name his first son in his honour.

Born in 1941, William grew up to be a handsome, clever, athletic young man. The first son of the Duke and Duchess of Gloucester, his lineage was impeccable, being George V's grandson and nephew of George VI. He was a page boy at the Queen's 1947 wedding to Prince Philip. 

Prince William pictured with Zsuzsi Starkloff in the Sixties: She was not considered an ideal royal bride at the time as she was Hungarian, twice divorced with a daughter

Yet as Giles St Aubyn, William's history teacher at Eton, recalls, he was torn between his sense of duty and his desire for adventure. 'He was fiercely intelligent and fought against too much of the royal life. He was allergic to being pushed around.'

After taking a degree at Cambridge, he joined the diplomatic service, a decision that allowed him to indulge his passion for flying, and when, in 1968, he was posted to Japan, he chose to fly there himself, piloting his light aircraft through gruelling storms for 16 days.

His derring-do created a sensation among the women of Tokyo – including Zsuzsi Starkloff. A glamorous 32-year-old former air stewardess, she had split from her second husband, a pilot, and was working as a model. 

Prince William of Gloucester pictured as a boy with his cousin Queen Elizabeth II (a princess at the time this picture was taken in 1944)

Having been introduced to the prince, then 27, at a dinner, she sent him a teasing note. 'My friend was giving a masquerade ball so I wrote on one invitation, "Dear Prince Charming. We heard a party is not a party without you, and besides I'm missing a slipper." I signed it "Cinderella".'

Her bid for his attention worked: William attended and came to her table asking if he could borrow Cinderella for a dance. 'That was the start of our love affair,' Zsuzsi recalls. 'Apart from work, we spent every minute together. He was so unspoiled, mature for his years.'

Yet clouds were looming: in 1969 William's cousin Princess Margaret arrived in Tokyo, purportedly on a trade visit. Was she there to scupper the relationship? 

'William told me he'd talked to her and she said she understood why he'd fallen in love, that I was pretty and smart,' she recalls. 'But I don't think she was my champion.'

Indeed, the documentary reveals that on her return to the UK Margaret wrote to her cousin, 'I was so pleased to have had a quick word with you. I do think you would be wise to wait for a bit, and then come home and see how everything looks.' 

Royal circles disapproved of the prince's love affair, he's pictured here in 1965 at a film premiere with the Queen Mother, Princess Margaret and Lord Snowdon

But William had other ideas. 'He said he'd written to the Queen to ask her permission to marry me,' Zsuzsi says. 'Elizabeth told him he should follow his heart but that Prince Philip was very unhappy with him.'

Despite the Queen's words, William found it increasingly hard to reconcile his feelings for Zsuzsi with his sense of duty. 'William had huge loyalty to his family, he had obligations and I knew he was struggling but I knew of his feelings for me and I had faith in the future. I never put pressure on him.'

Fate intervened when in 1970, William's father Prince Henry had a stroke and he had to return to England. But if the courtiers thought this was the end of the affair they were wrong - he soon invited Zsuzsi, who'd moved to New York, to join him at the family's country estate, where she met his parents. 

'His father was very ill and in a wheelchair. But I had a wonderful welcome from the duchess. She was warm and friendly, sitting with her flowers and her needlework, and we chatted. But she was very reserved and it was hard to know what she was really thinking.'

Prince Charming: William of Gloucester pictured with Susan Osborne at a Ski Ball at Watford

Zsuzui Starkloff, pictured arriving at Heathrow in the Sixties said William gave her beautiful memories 

A month later, however, the prince put the affair on hold. 'There was never a formal goodbye,' Zsuzsi says. 'He needed to work through whatever he needed to work through, but I believed that at some point we'd be back together.'

She flew back to New York, but the lovers continued to write to each other and talk on the phone. In the summer of 1972, Zsuzsi says, William invited her to England to join him in the cockpit for an air race. 'I said I couldn't because I had plans. But we talked about him coming to New York so we could talk properly again.'

It was not to be: on 28 August 1972, the prince was killed when his light aircraft crashed in that air race at Wolverhampton. Zsuzsi learned about it from a journalist who phoned her at home in New York. 'I was devastated,' she says. 'My first thought was for his mother, who was in the audience - how terrible to watch your son die.'

The prince, pictured flying a plane in 1972, died months later in a crash during an air race

Her tribute was to live her life in the way she felt he would have wanted. 'William encouraged me to learn to fly,' she recalls. 'So when we lost him I not only became a pilot, but a flight instructor.' She never married again, although she has a long-term partner.

Just six years after William died, Prince Michael of Kent married Czech divorcée Marie-Christine von Reibnitz with the approval of the court, and Princess Margaret herself became the first senior royal since Henry VIII to get divorced. 

Yet Zsuzsi refuses to dwell on what might have been. 'If William had been born in a different age he'd have been free to pursue his heart. But we gave each other beautiful memories, and not many people can say that.' 

The Other Prince William is on Thursday 27 August at 9pm on Channel 4.