A forbidden romance and a life cut short: The story of 'the other Prince William'.

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Today, Prince William, Duke of Cambridge, is one of the most popular members of the royal family.

Sixty years ago, however, the situation could not have been more different for Prince William of Gloucester, the man who inspired the name of today’s well-known Prince William, and who lived an unorthodox royal life filled with twists and tragedy.

Born in 1941, Prince William was the son of Prince Henry, brother of King George VI. He is the first cousin of Queen Elizabeth, and at six-years-old served as one of the two page boys at her wedding to Prince Philip in 1947.

In line with family tradition, Prince William attended Eton College in 1958, before studying history at Cambridge University, where he graduated with a Bachelor of Arts degree in 1963. He then became the first royal to attend an American university, when he went on to study business and politics at Stanford University.

From an early age, Prince William, who was once forth in line for the throne, seemed determined to live outside of the strict constraints and conventions of the monarchy.

Following university, Prince William forwent the expected military career for men in the royal family, and instead chose a profession in diplomatic service.

In 1968, Prince William was transferred to Tokyo, Japan, as second secretary in the British Embassy.

There, Prince William met a woman named Zsuzsi Starkloff, who was a Hungarian mother and model, who also happened to be twice-divorced.

In 2015, Starkloff told a British television documentary how the pair came to meet.

"A friend was giving a masquerade ball. I said it would be fun to invite Prince William. We had an idea, to write a handwritten invitation... It said, 'Dear Prince Charming, we hear a party isn't a party without you. And besides that, I'm missing a slipper,' I signed it Cinderella," she remembered.

"I was dressed as an Indian princess and he dressed as the lone ranger with a black cape and a mask. It was really fun. He was obviously very good looking and tall, more like the movie James Bond than Sean Connery… he came over to our table and he said, 'May I borrow Cinderella for a dance?' And we danced, and that's when our relationship truly began."

The couple's relationship blossomed as they quickly developed a passionate affair, far away from the prying eyes of Prince William's home in England.

Watch: Zsuzsi Starkloff reveals how she met Prince William. Post continues below video. 

Three months into their relationship, Starkloff remembers Prince William telling her: "I never thought love could be this beautiful".

"It's a true fairy tale," Starkloff reflects. "Long ago and far away; a love story between Prince William and a Hungarian girl in Tokyo, Japan.

"It was amazing, we had such different backgrounds – different families, different countries, different language – but we just clicked and we never had arguments. We were just two people happy to be together."

The couple moved in together, renting a house near the Pacific Ocean.

But as easy as their relationship seemed, thousands of kilometres away, it was frowned upon by Prince William's family.

When news of their romance reached the walls of Buckingham Palace, the royal family swiftly came to intervene.

In September 1969, William's cousin and Queen Elizabeth's sister, Princess Margaret, made a visit to Tokyo ostensibly to improve trade relations between the countries. Though she also wanted to talk to her cousin about his new affair.

"She and William talked quite a bit. I was very curious of what she would say. I didn't ask William, all he volunteered after the long conversation she had with him, was that she said I seemed very nice and interesting, and she didn't blame him for falling in love with me," Starkloff revealed.

Upon returning to the UK, Princess Margaret wrote a letter to Prince William, advising him to not make any "rash decisions" about his relationship.

Ultimately, Prince William, as a potential successor to the throne, would need the Queen's approval and permission to marry his partner by law. Given Prince William's choice, such approval didn't seem likely.

"My relationship with William had nothing to do with his title or me wanting to be princess. That was not what it was about, and William knew that," Starkloff said.

In 1970, Prince William's father suffered a stroke and he was summoned back to England. He decided to bring his girlfriend, Starkloff, back with him to meet his extended family. Unsurprisingly, the royal family did not approve.

"Prince Philip was pretty much up in arms against us marrying," Zsuzsi recalls. "I didn't like to see William struggle... I believed that he wanted to stay together with me. I know he felt the weight of responsibility on his soldiers."

In a rare TV interview, Prince William explained: "I was perhaps two very different people. In one respect, Prince William, as a member of the family, and treated in that way. And in the other respect, as a private individual who had his own thoughts and ambitions. And, at times, I would behave completely differently."

Eventually, Prince William made the heartbreaking decision to part ways with Starkloff in 1970. Though, the two kept in touch and spoke of a hopeful future where they would be together.

In August 1972, Prince William invited Starkloff to fly with him in an air race, though she wasn't able to change her schedule.

William still went, flying alongside his co-pilot Vyrell Mitchell. Within minutes of taking off, their plane crashed to the ground and the two were instantly killed. Prince William was just 30-years-old when his life was cut tragically short.

Then 23-years-old, Queen Elizabeth's first son, Prince Charles, was devastated at the loss of his cousin, who many said was an idol for Charles. Many years later, Prince Charles would name his first son in honour of his cousin.

Prince William of Gloucester died 10 years before Prince William, Duke of Cambridge, was born.

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