The Prince and Princess of Wales are preparing to celebrate their 13th wedding anniversary at the end of the month.
And as they approach this milestone, they can perhaps reflect on the fact that their "unshakeable" bond has only grown stronger.
The couple are facing their biggest challenge yet – Kate's preventative treatment for cancer – but the Princess' health struggles have brought them closer together, and they will emerge stronger as a result of their experience, according to royal author Robert Jobson.
© GettySpeaking exclusively to HELLO! in this week's issue, Robert, whose book Catherine, the Princess of Wales: A Biography of the Future Queen is published on 4 July, said: "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."
Robert also added: "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.
"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."
© GettyThe 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.
The pair may now be a world away from the carefree early years of their love story, but the Prince is as devoted to his wife as ever and determined to be there as they navigate her treatment for cancer together.
© GettyAlthough their responsibilities to the Crown and royal duties will increase over time, as they prepare for their destiny as King and Queen, the couple are choosing to prioritise their family life right now and to keep things as normal as possible for their beloved children Prince George, ten, Princess Charlotte, eight, and Prince Louis, who turns six on Tuesday.
The heir to the throne did the daily school run while Kate was recovering from abdominal surgery in January, and in recent weeks she is thought to have been keen to join them when she can.
"Catherine has done her best to stick to a routine for the sake of the children," Robert said. "She and William know the importance of taking each day as it comes. It was a huge blow when they first learned of the diagnosis, but they are both mentally strong.
"Without doubt, it has brought the couple even closer together."
To read the full interview, pick up the latest issue of HELLO! on sale in the UK now. You can subscribe to HELLO! to get the magazine delivered free to your door every week or purchase the digital edition online via our Apple or Google apps.
