Kate Middleton and Prince William's 13th wedding anniversary will be "different" from previous celebrations as the Princess of Wales continues her cancer treatment, according to a royal expert.

Monday will mark 13 years since Prince William and Princess Kate's royal wedding at London's Westminster Abbey on April 29, 2011.

Royal commentator Richard Fitzwilliams said he believes the couple will celebrate the occasion "privately" due to Middleton's condition.

"This year will be very different from previous years as Catherine is undergoing preventative chemotherapy after recovering from abdominal surgery," Fitzwilliams was quoted as saying by GB News.

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"The princess has appealed for time, space, and privacy at this difficult time, and there is unlikely to be a public commemoration of any kind," he added.

The royal expert told the U.K.'s The Sun that this year's wedding anniversary will not be "easy" for Prince William and Middleton, the latter of whom went public with her cancer diagnosis last month.

Prince William, Prince of Wales, and Catherine, Princess of Wales, Kate Middleton
Britain's Prince William, Prince of Wales (L) and Britain's Catherine, Princess of Wales attend a pre-campaign launch event, hosted by The Royal Foundation Centre for Early Childhood, at BAFTA on January 30, 2023 in London, England.
(Photo : Eddie Mulholland - WPA Pool/Getty Images)

"All sorts of memories will come back" for the couple, Fitzwilliams predicted.

He suggested that Prince William and Middleton would likely spend the day with their children, Prince George, Princess Charlotte, and Prince Louis, and other family members.

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"It's been a very difficult year, and clearly, they're [a] very close-knit family. They will rejoice in each other's company. They will involve the family," Fitzwilliams told the outlet.

But he said the Prince and Princess of Wales could continue the wedding anniversary tradition they have been doing in recent years: posting a photo of them on social media to mark the occasion.

"It does [look] as if we'll probably get a photograph of some sort or another," Fitzwilliams told The Sun, recalling the "delightful" snap shared by Middleton and Prince William last year.

For their 12th wedding anniversary, the future king and queen posted on Instagram and X, formerly Twitter, in 2023 a sweet snap of them riding bikes in Norfolk, England.

Prince William and Middleton had their arms around each other as they smiled for the photo, which was taken by photographer Matt Porteous in 2022.

A post shared by instagram

The royal couple also marked their milestone 10th wedding anniversary in 2021 with romantic portraits.

British photographer Chris Floyd captured Prince William embracing and lovingly gazing at a smiling Princess Kate in one snap. Another photo showed Middleton leaning her head on her husband's chest as they smiled for the camera.

To celebrate nine years since their wedding in 2020, Middleton and Prince William posted a throwback photo from their lavish 2011 ceremony.

In the picture, the prince and princess can be seen holding hands as they left Westminster Abbey following their wedding.

Middleton has not been seen publicly since she shared a video message in late March announcing that she was diagnosed with an undisclosed form of cancer following her abdominal surgery in January.

No additional information has been disclosed about her cancer battle since the video.

Prince William, for his part, has been juggling royal engagements with caring for his wife and children amid Princess Kate's chemotherapy.

On Thursday, the heir to the British throne received gifts for his wife during a visit to St. Michael's Church of England High School.

Among these gifts were Lego sunflowers and a crocheted starfish, the school's senior mental health lead Kerry Whitehouse told People.

Whitehouse shared that Prince William "thanked" them for the presents and told them "the Lego would go down well in the house."