Prince Charles' whirlwind romance with Diana's older sister revealed

Princess Diana and Prince Charles might have had a famous marriage (in the end for the wrong reason), but before the pair tied the knot he dated Diana's older sister Sarah Spencer first.

Charles was 32 when he married Lady Diana Spencer, and as the world's most eligible bachelor, had been linked to several women over the years.

There was the then Camilla Shand (now his wife the Duchess of Cornwall) who would go on to marry Andrew Parker Bowles and, for a short time, Diana's older sister – Lady Sarah Spencer.

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Sarah was 22 and was seen as being a suitable match for the future king, coming from an aristocratic family background – the daughter of an earl.

It was Charles' relationship with Sarah that led to his first meeting with a very young Diana, during a hunting trip to the Spencer family's grand country estate of Althorp in November 1977.

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Charles was 29 while Diana was only 16.

Sarah actually calls herself Cupid for introducing them, after she was outwardly accepting of their relationship.

"I introduced them. I'm Cupid," the Guardian reported her as saying when Diana and Charles announced their engagement.

So what happened between Sarah and Charles that ended their relationship?

It's simple. Sarah wasn't in love with him.

After spending a weekend at Windsor Castle, Sarah bluntly said to reporters, "Charles makes me laugh a lot. I really enjoy being with him," but then added "there is no chance of my marrying him. I'm not in love with him".

"And I wouldn't marry anyone I didn't love whether he were the dustman or the King of England," Sarah said. She also called Charles a "romantic who falls in love easily".

Unfortunately, Sarah also talked to the press about their relationship, a move that apparently infuriated Charles.

When Sarah finally confessed that a story full of her quotes was about to come out, royal biographer Tina Brown claims he said to her coldly: "You've just done something extremely stupid."

Sarah went on to marry Neil Edmund McCorquodale and the pair have three children.

Diana apparently told her friends that she would marry Charles because he was "the one man on the planet who is not allowed to divorce me," according to The Diana Chronicles.

Diana spoke to author Andrew Morton, who wrote Diana: Her True Story — In Her Own Words about how Charles proposed to her.

"He said: 'Will you marry me?' and I laughed. I remember thinking, 'This is a joke,' and I said: 'Yeah, OK,' and laughed. He was deadly serious. He said: 'You do realise that one day you will be queen.' And a voice said to me inside: 'You won't be queen, but you'll have a tough role'. So I thought 'OK,' so I said: 'Yes'. I said: 'I love you so much, I love you so much.' He said: 'Whatever love means'."

Charles didn't even have an engagement ring. Instead, Diana chose her own ring from a Garrard catalogue.

It was Diana who fell in love with the 12-carat sapphire ring, surrounded by 14 solitaire diamonds set in 18-carat white gold.

It cost Charles $37,000. Today, the ring is worth $500,000 and is worn by their son Prince William's wife, the Duchess of Sussex.