Kate Middleton went so far as to rewear a dress from her closet in order to keep her return to the public eye a secret — while making contingency plans if she felt ill on the day, sources tell Page Six.

The cancer-stricken Princess of Wales was keen to join her family at King Charles’ birthday celebration and Trooping the Colour parade this past Saturday.

But there were only a few people kept in the loop. Kate personally called her father-in-law — who’s also being treated for cancer — to tell him the good news and he was “delighted” she could attend, the king’s spokesperson told Page Six.

Last year, Charles, 75, stood next to his oldest son and heir to the throne, Prince William on the balcony at Buckingham Palace. This year, Kate was placed next to the monarch, in a clear show of just how much they have come to rely on each other, according to a palace insider.

Behind the scenes, only a few trusted people knew of Kate Middleton’s plan to attend the Trooping the Colour parade – and arrangements were made in case she felt ill on the day. Zak Hussein / SplashNews.com

Kate Middleton made a triumphant return at Saturday’s Trooping the Colour parade amid her cancer battle. She’s seen here on the balcony of Buckingham Palace with Prince William, Prince George, Princess Charlotte and Prince Louis. Zak Hussein / SplashNews.com

Well aware that her appearance would cause a commotion around the globe, 42-year-old Kate — who had not made a public appearance since Christmas Day — quietly called upon royal photographer Matt Porteous for a top-secret shoot at home last week.

Porteous had previously been trusted by the Waleses to take the photos at Prince Louis’ christening in 2018, as well as the shot that was used for the family’s 2022 holiday card. He’s even caught Prince William, 41, and Kate on camera as they went shark diving in Belize.

Kate penned an extremely personal message to be issued alongside her latest photo, admitting there are “good days and bad days” and stressing that she is “not out of the woods yet”

Kate could not help but smile as she traveled down The Mall in a carriage with her children. AP

Dressed down in jeans, a white T-shirt, brown blazer and sneakers, the princess was captured by Porteous soaking up the lush greenery of the Windsor estate close by her home, Adelaide Cottage.

Being outside gives her strength and “being connected to nature is something that continues to support her recovery,” a source told us. “She enjoys spending personal time on the things that give her energy and positivity.”

Kate stood right next to King Charles — who is also battling cancer — as well as William and her children Louis and Charlotte. TOLGA AKMEN/EPA-EFE/Shutterstock

William took pride of place next to his father at last year’s Trooping The Colour parade. UK Press via Getty Images

When it came to Kate’s outfit for the parade, she chose to “upcycle” a white dress she first wore on the eve of King Charles’ coronation last year, with her hair swept into an updo beneath a white hat by Philip Treacy.

She accessorized the outfit with her Cassandra Goad Cavolfiore pearl studs — previously worn to Prince Louis’ christening in 2018 and Wimbledon in 2023 — and the Irish Guards Regimental brooch, nodding to her role as the regiment’s honorary colonel.

The simple white and black dress by British designer Jenny Packham was updated with black trim and ribbon embellishment on the neckline, as well as a black belt.

Kate secretly asked photographer Matt Porteous to come to Windsor last week to take a photo of her within the grounds. Sources said that being in nature has helped with her recovery. via REUTERS

While repeating the dress made a sustainable fashion statement, it also served another purpose.

“Just by reworking what she already owns, it allowed Kate to keep this secret [of her appearance] within her very closest and most trusted team,” said Bethan Holt, Fashion Director at the UK Telegraph.

Kate is still believed to be working with her longtime stylist Natasha Archer.

“We definitely know that Kate has tailors in house to help make something fit her perfectly, and they could have easily added the new belt and that bow detail at the neckline, so Jenny Packham may not have even known Kate was going to re-wear the dress,” said Holt.

Kate “upcycled” a white dress by Jenny Packham for her return to the spotlight. Experts said this allowed her to keep her appearance a secret. James Veysey/Shutterstock

However, Holt added, Packham is one of Kate’s “most trusted” designers.

She pointed out that Kate has worn a custom Packham dress every time she left hospital with each one of her newborn children.

“Those are very intimate, important moments, so it’s a natural decision that Kate would turn to Jenny once again,” said Holt.

When it came to Saturday, behind the scenes, staff at both Buckingham Palace and Kensington Palace worked together to ensure there would be appropriate contingencies if Kate felt unwell or unable to travel to the Horseguards Parade and step on the Buckingham Palace balcony.

The Princess of Wales first wore her Jenny Packham dress at a reception on the eve of King Charles coronation last May before having it altered for Saturday. Getty Images

Saturday marked Kate’s first public event in six months since she announced in March that she has cancer and is undergoing chemotherapy.

She could have easily rested inside Buckingham Palace while her children traveled with another senior member of the royal family, such as the Duchess of Edinburgh.

But in the end, Kate was able to travel by carriage in the Glass Coach with Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6, before they joined Prince William on the balcony.

Looking happy to be back Saturday, Kate was seen tending to her children and talking to Princess Charlotte. ZUMAPRESS.com

Palace sources were keen to stress that although we may see her at a few upcoming appearances, it does not mean that she is back to work full time.

“My treatment is ongoing and will be for a few more months,” Kate wrote with her photo. “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.”