Marching for color – live: Kate Middleton photographed at Buckingham Palace in her first appearance since being diagnosed with cancer

The Trooping the Color procession, which will celebrate the King's official birthday, has begun with members of the Irish Guards marching along The Mall.

The British Army has shared a video on social media of troops preparing for Trooping the Colour, one of the highlights of the royal calendar.

Political artist Kaya Mar was among the spectators gathered to watch the royal procession down The Mall.

The 67-year-old woman had brought two oil paintings, one of the King and another of the Princess of Wales, her husband and their children.

Mar said: “The royal family is like an anchor in times of uncertainty, especially when you see the messiness of politicians.”

He added: “They are the only stable political institution in the midst of turmoil in the UK and, really, what is the only stable institution for the future of this country?

“It's still royalty. They are harmless. They are not politicians. Whether you're on the right, the left, the middle, whatever, you admire them.”

The artist said his painting of Kate, which invoked biblical imagery by comparing her to Mary, was intended to mock the “religious fervor” he thought surrounded the birth of his most recent son, Prince Louis, in 2018.

The Princess of Wales has been seen arriving at Buckingham Palace in a car ahead of Trooping the Colour.

A brief glimpse of Kate showed her in a pale dress with a hat, and the princess was expected to appear on the balcony alongside the Royal Family.

In a statement, the Princess of Wales said: “I have been overwhelmed by all the kind messages of support and encouragement over the past few months. It has really made a big difference to William and I and has helped us both through some of the most difficult times.

“I'm making a lot of 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 letting your body rest. But on good days, when you feel stronger, you want to make the most of feeling good.

“My treatment is ongoing and will be for a few more months. On days when I'm feeling pretty good, it's a pleasure to get involved with school life, spend personal time doing the things that give me energy and positivity, as well as start working a little from home.

“I'm looking forward to attending the King's Birthday Parade this weekend with my family and hope to join in on some public engagements over the summer, but I also know I'm not out of the woods yet.

“I am learning to be patient, especially in the face of 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 who have so bravely shared their stories with me.”