LEANING ON THEIR LOVE IN TOUGH TIMES THE PRINCE AND PRINCESS OF WALES ARE CLOSER THAN EVER AS THEY PREPARE TO MARK THEIR ANNIVERSARY

It has been 13 years since they exchanged vows at the altar of Westminster Abbey, promising to face together whatever came their way, "in sickness and in health".

And as they prepare to celebrate their wedding anniversary on 29 April, the Prince and Princess of Wales can reflect on the fact that their "unshakeable" bond has only grown stronger.

As they face what is perhaps the couple's biggest challenge yetKate's treatment for cancer - they have become closer than ever, and they will emerge stronger as a result of their experience, according to royal author Robert Jobson.

"It has been tough on both of them, not least as parents, as they try to reconcile their children's mental wellbeing and the seriousness of her diagnosis," he says.

"But there is no question that William has been Catherine's rock. He is the constant in her life and she is at the heart of his."

Robert, whose book Catherine, the Princess of Wales: A Biography of the Future Queen is published on 4 July, tells HELLO!: 

"Their forthcoming anniversary is a testament to not only their love, but also their unity. I'm sure they'll acknowledge their journey, knowing that their strength lies in being together.

"The ups and downs they have weathered together have cemented their bond, which is now unshakeable." The royal couple met while students at the University of St Andrews and became close friends before romance blossomed in 2002.

It has now been 20 years since news of their relationship went public, after they were photographed together on the slopes at Swiss ski resort Klosters.

CERTAIN OF THEIR LOVE

Their courtship was not without its challenges, and they split up for three months in 2007 before getting back together and becoming more committed than ever.

But William, who experienced the breakdown of his parents' marriage, waited until 2010 to propose, explaining later that he had wanted to give Kate "a chance to see in and to back out, if she needed to, before it all got too much".