Catherine Middleton: Princess Kate says her treatment is ongoing but she will be at Trooping today

The Princess of Wales has penned a moving personal message about her cancer battle, revealing she will make her first public appearance since Christmas at today’s Trooping the Colour.

Kate, 42, says her “preventative chemotherapy” treatment is continuing and, while making “good progress”, she is “not out of the woods yet.”

With touching candour, the future queen admits that like anyone undergoing cancer treatment, she has “good days and bad days”.

At her worst, she writes, she feels “weak, tired” and just wants to rest.

But on her good days, she is determined to make the most of feeling well, getting involved with her children’s school life and starting to do a little work from home.

And while she is not ready to return to public duties full-time, the princess says she does feel well enough to undertake a small number of public engagements over the summer.

The first will be an appearance at the King’s official birthday parade today, where the princess will ride in a carriage with her three children, Prince George, Princess Charlotte and Prince Louis, down The Mall before watching the spectacle from the Major General’s Office at Horse Guards with the rest of the Royal Family.

Her husband, Prince William, will accompany his father, King Charles, who is undergoing his own cancer treatment and will also travel in a carriage, on horseback.

It is hoped Kate – who is set to receive a joyous reception from the crowds – will also feel strong enough to join them on the Buckingham Palace balcony for the traditional public appearance and fly-past.

All plans are subject to how well she feels on the day. It is understood the princess spoke to the King in advance about her attendance, with a Buckingham Palace spokesman saying His Majesty was “delighted”.

It is thought she chose to dip back into the public spotlight at Trooping because family is so important to her and she wanted to support her husband and children.

To mark the positive news, Kensington Palace released a stunning new portrait of the princess taken at Windsor earlier this week by one of her most trusted personal photographers, Matt Porteous, showing her looking radiant and reflective.

Wearing a simple linen jacket, jeans and trainers, the princess leans against a tree, looking up to the skies, as if gazing optimistically into the future.

It is understood the choice of background was no coincidence, given the comfort she has taken over recent months from the things she loves, including being outdoors in nature with her family.

In her statement, the Princess says she has been “blown away” by all the messages of support and encouragement she has received. The princess announced that she had cancer on March 22.

In an unprecedented video statement, also recorded at Windsor where the family live, she revealed that she had been diagnosed as a result of post-operative tests following serious abdominal surgery in January and was undergoing “preventative” chemotherapy.

She revealed the news had come as a “huge shock” and said that she and William “have been doing everything we can to process and manage this privately for the sake of our young family.”

Kensington Palace said at the time that it would not be sharing details of what kind of cancer the princess had and asked people not to speculate.

Last month the Mail exclusively revealed that the princess had been deluged with thousands of letters, cards and gifts from well-wishers.

It is understood that she is reading as many as she can, and has been touched to read so many deeply personal stories. Sources say she and her family are also incredibly grateful to members of the public and the media who have afforded her the time, space and privacy that she asked for in March.

While her attendance at Trooping the Colour will undoubtedly be a “significant moment”, it has been strongly stressed that Kate’s appearance should not be seen as a return to a full schedule of public engagements quite yet.

However it can be revealed that she has started to work a little from home at Adelaide Cottage in Windsor, and even holding meetings with her private secretary and senior staff.

In addition to briefings from her team at Kensington Palace, The Royal Foundation and the Centre for early Childhood, the princess has also started to undertake meetings relating to her work and projects.

From now on, these will be documented in the Court Circular, the official record of royal engagements.

Asked how the princess was feeling, a Palace spokesman said: “As she has said she is making good progress and is looking forward to joining the family tomorrow.”

The spokesman said they would not be commenting further on her treatment and how long it is expected to last.

Sources close to the princess also renewed their appeals for people to respect her privacy as she continues her treatment, giving her time and space with her young family.

They said she must be allowed to continue to “prioritise” her recovery, stressing that some recent reports about her progress, exacerbated by speculation on social media, have been distressing and unhelpful to her recovery.

Prime Minister Rishi Sunak said it was “wonderful news” that the princess is to join the rest of the royals today.

At the G7 summit in Italy, he said: “The whole country will be behind the Princess of Wales for her recovery.”

”I saw her statement, I thought it was brave, I thought it was honest and I’m sure it will bring an enormous amount of comfort to so many other people who are grappling with similar health challenges, so I think she deserves enormous praise for what she said.”