Following their shock split in 2007, when Kate and Wills chose to rekindle their romance they made an important pact.
They made a secret agreement to marry each other so Middleton had some certainty after they had been dating for years and weren't yet engaged, reports The Sun.
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According to royal expert Katie Nicholl, after getting back together after a few months being apart, the couple made the promise to one another.
Nicholl's new book "Kate, The Future Queen," reveals that while holidaying on the island of Desroches, in the Seychelles off East Africa, the agreement was made.
She writes: "There on the paradise island, William promised Kate that he was in the relationship for the long term.
"For the very first time, they talked seriously about marriage, and with the ocean before them and beneath the night sky, they made a pact to marry."
The book explains that Prince William promised Kate a diamond ring, with the intention to, "take the pressure off and just promise we'll end up together".
According to The Sun, Wills spoke about their split a long time after and said: "We were both very young.
"We were both defining ourselves as such and being different characters.
"It was very much trying to find out way and we were growing up, it was just a bit of space — things worked out for the better."
Middleton continued: "I think I … at the time wasn't very happy about it, but actually it made me a stronger person.
"You find out things about yourself that maybe you hadn't realised, or I think you can get quite consumed by a relationship when you are younger and I really valued that time for me as well although I didn't think it at the time."
After meeting at the University of St Andrews in 2001, the pair were together by 2003.
In 2007 they briefly split, but only for a few months.
In 2010 the couple was officially engaged and married the following year. They now share three children, Prince George, Princess Charlotte and Prince Louis.