Major protocol change King Charles made to Kate Middleton during Trooping The Colour that you may not have noticed

A Royal expert has revealed the protocol change King Charles made to Kate Middleton at Trooping the Colour this weekend.

While many were unsure whether the Princess of Wales, 42, would attend the King's birthday event due to her ongoing cancer treatment, Kate appeared alongside Prince William and their three children - Prince George, 10, Princess Charlotte, nine and Prince Louis, six.

There was a protocol change at Trooping the Colour (Max Mumby/Indigo/Getty Images)

Kate was pictured arriving at the ceremony, as well as joining other senior members of the Royal Family on the balcony of Buckingham Palace.

The Royals gather on the balcony during the event to watch the RAF flyover.

It was during this time that King Charles made a touching gesture towards his daughter-in-law, in what is a huge change of protocol.

During the flyover, Kate was seen next to Charles, a change from last year's event when Prince William was pictured next to his father.

Of course, it's not clear if the move was intentional, but with Kate and Charles laughing and joking together, the Princess of Wales seemed at ease next to the monarch.

Grant Harrold, a former royal butler, has said that as per royal protocol, the Prince and Princess of Wales would traditionally stand together, while King Charles and Queen Camilla would also be next to one another.

"The protocol is that traditionally Kate and William would be standing next to each other and the King and Camilla standing next to each other," Grant said on behalf of Spin Genie.

"It is planned but it is also changing the 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 is all orchestrated but I have a feeling the King said to Kate ‘you stand next to me’."

Last week, Kate provided an update on her health.

In a statement posted on social media on Friday 14 June, the princess opened up about her health.

She wrote: "I have been blown away by all the kind messages of support and encouragement over the last couple of months.

"It really has made the world of difference to William and me and has helped us both through some of the harder times.

"I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days. On those bad days you feel weak, tired and you have to give in to your body resting.

"But on the good days, when you feel stronger, you want to make the most of feeling well. My treatment is ongoing and will be for a few more months.

"On the days I feel well enough, it is a joy to engage with school life, spend personal time on the things that give me energy and positivity, as well as starting to do a little work from home."

Fans were overjoyed to see Kate at the event (Max Mumby/Indigo/Getty Images)

The Princess continued: "I’m looking forward to attending The King’s Birthday Parade this weekend with my family and hope to join a few public engagements over the summer, but equally knowing I am not out of the woods yet.

"I am learning how to be patient, especially with uncertainty. Taking each day as it comes, listening to my body, and allowing myself to take this much needed time to heal.

"Thank you so much for your continued understanding, and to all of you who have so bravely shared your stories with me."