Kate Middleton and Prince William are 'going through hell,' says 'heartbroken' confidante

Arrieta, who has worked as a stylist for the Prince and Princess of Wales' three children since Prince George was a baby, has shared a worrying update on the couple following Middleton's cancer diagnosis.

"I'm heartbroken at the moment, I think they are going through hell, I hope they will be back. It's really personal," Arrieta told the Telegraph.

The children's clothing designer, who owns her eponymous label Amaia, has refrained from revealing any nuggets of information about the Waleses until now.

The Post has reached out to Kensington Palace for comment.

While it's unclear what made her share the concerning update as the princess continues to receive chemotherapy for her condition, Arrieta's remarks could indicate the seriousness of the royal's condition.

Elsewhere, Arrieta opened up about her role as the stylist for Prince George, Princess Charlotte and Prince Louis.

"Dressing the royals gives me so much pride, I really do care about how they look," she told the outlet.

Arrieta said she works closely with the children's current nanny, Maria Borrallo, on dressing the trio.

"It's often a very tight deadline. We would do anything for them … the children always look amazing in the end," she added.

Prince Louis was dressed in an Amaia shirt in the image released by the Waleses for his sixth birthday last week.

Middleton, 42, is in the thick of her cancer treatment following her diagnosis in March.

She's not the only royal dealing with health woes, as King Charles, 75, revealed his cancer diagnosis in February, and Sarah Ferguson, the Duchess of York, is battling an aggressive skin cancer.

It's understood that Middleton decided to share the news with the world on March 22 as it was before she and William's three children left school for Easter break.

The Post revealed that the Waleses "sugarcoated" her health woes when breaking the news to their youngest child, Louis, but had a more "difficult" conversation with George and Charlotte.

As the royal undergoes treatment, her parents, Michael, 74, and Carole Middleton, 69, have stepped in to support their grandchildren during the fraught time.