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.