There is nothing like a shared experience to bring two people even closer together. Without a doubt, King Charles and the Princess of Wales will have found strength in each other as they confront their own respective health battles.

That bond has never been more in evidence than at Trooping the Colour in mid-June. In a break with protocol, King Charles arranged for his “darling daughter-in-law” to stand next to him as the family gathered on the Buckingham Palace balcony to wave to the crowds. Their closeness was clear to see as Kate whispered something to Charles, making him laugh before the National Anthem was played.

Last year, Charles stood next to his son William, the Prince of Wales. The change of order was viewed as a mark of respect for Kate and an acknowledgment that she had chosen his official birthday to return to the public eye.

Former royal butler Grant Harrold exclusively told OK!, “It is changing protocol to show the King and Kate united with each other and going through similar battles, so I would say this is down to the King’s preference. The balcony [line-up] is orchestrated, so I have a feeling the King said to Kate, ‘You stand next to me.’”

The timing of the pair’s illnesses has been eerily similar. In January, both Kate and Charles were treated in the same hospital, the London Clinic, at the same time and were able to offer reassurance to one another in the early stages of their treatment. Then, the announcement of their cancer diagnoses came just a month apart, Charles’ in February and Kate’s in March.

After Kate’s public video message on 22 March, in which she revealed her diagnosis to the world, the King released a statement saying that he was “so proud of Catherine for her courage in speaking as she did”. It added that the King was in “the closest contact with his beloved daughter-in-law” and that both he and Queen Camilla “continue to offer their love and support to the whole family through this difficult time”.

It was a remarkable public expression of their private fondness, with royal expert Hugo Vickers telling us he would not be surprised if there hadn’t been “a lot of plodding along the hospital corridor to see each other”.

Kate and Charles have been leaning on each other amid their respective cancer diagnoses ( 
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WireImage)
Kate’s closeness to Charles has long been recognised in the royal family, and she has even been overheard calling him “Grandpa”. The strength of that relationship is due to the fact that in the early stages of William and Kate’s relationship, Charles made a special effort to bring his son’s girlfriend into the family fold.

The princess has described her father-in-law as being “very, very welcoming, very friendly”. In 2004, she was invited to Charles’ 56th birthday party at Highgrove. There were also invitations to several family ski trips to Klosters – where Kate and William were first pictured together – and in 2008, the fact Kate was included on another ski trip showed that she was truly back in the fold after her brief split from William.

Royal expert and author Duncan Larcombe says, “Charles and Camilla adore Kate, and I think they have from the very beginning. One of the very early signs of that was the fact that William got permission for Kate to come on Charles’ ski trip with the family in 2004 and 2005. Charles wouldn’t have done that if he didn’t approve of her then.

Charles said at William and Kate’s wedding that the family were “lucky to have her” ( 
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Getty Images)
“That was the start of this bond between Charles and Kate. He’ll be absolutely over the moon with how she’s grown into the role, how she’s made it her own and how the public have warmed to her.”

In her book Kate: The Future Queen, royal author and biographer Katie Nicholl writes, “William’s father was very fond of Kate and already saw her as a future daughter-in-law.”

Charles backed this up in practical ways, too, allowing Kate to use his lawyers Harbottle & Lewis when she was besieged by paparazzi in the London years. He also offered up his house, Highgrove, as a bolthole for the young couple so they could spend time together privately – perhaps mindful of how much scrutiny the late Diana, Princess of Wales, had come under in the early stages of their relationship.

In Katie’s book, a senior member of Charles’ household is quoted as saying, “Charles was like any other father: he was very kind and hospitable towards Kate from the beginning. He was delighted William had found such a lovely girlfriend, and she was made to feel at home.”

Unlike Diana, the young Kate took to country pursuits like deer stalking and shooting immediately. In 2007, Kate and Charles were pictured, without William, in what appeared to be a lesson in the art of the sport.