The effects of Kate Middleton’s cancer battle are being felt throughout the British monarchy.

Patrick Jephson, the former private secretary to the late Princess Diana, told Vanity Fair that the House of Windsor hasn’t been the same since the future queen consort stepped away from her royal duties to focus on her health.

“Without Catherine, it all seems rather flat,” Jephson said in the cover story published Tuesday. “The future of the monarchy is William and Catherine.”

“And, as we know from any superficial study of the British royal family, it’s the women who pull the show together, who get out there and make things happen,” he added. “So how fragile is the monarchy? Well, it’s as fragile as Catherine is, and at the moment, we don’t know.”

Middleton, 42, announced her diagnosis in March — mere weeks after King Charles III revealed that he also has cancer.

The 75-year-old monarch has carried on with his royal duties, while Middleton removed herself from the spotlight, although she did attend the Trooping the Colour ceremony on June 15.

“The intense public emotion that greeted Catherine’s brave appearance at Trooping underlined her crucial importance to the monarchy,” Jephson said about the Princess of Wales.

He continued: “Without her, the institution would surely be reduced to a shadow of itself. Given the Windsors’ drastically thinned ranks, she is the crown’s best hope by far. She combines duty and beauty with a piercing vulnerability second only to Prince William’s mother.”

Doctors discovered Middleton’s cancer during a planned abdominal surgery in January.

Middleton and William’s children Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6, didn’t know about their mom’s diagnosis at first, but according to an aide, Middleton “was determined to be honest with them and told them in a way they could understand.”

The princess ended up telling the world about her health woes in a prerecorded video on March 22 — only after speculation began swirling about why she had been noticeably absent from public view.

“It was her decision to share her message with the nation about her journey,” a long-standing aide told Vanity Fair. “That is an incredible example of the inner strength that she has as an individual. It shows a huge amount of courage, and I think a lot of people have taken inspiration from that.”

As the mother of three continues her chemotherapy sessions, those familiar with the Wales’ said that the family are having a “terribly hard time,” but Middleton is not giving up.

“Catherine has an inner strength, she’s a strong person and she will be drawing on that,” Middleton and William’s former private secretary, Jamie Lowther-Pinkerton, said to VF. “That strength has been very useful when there has been turmoil in her life.”

A friend also noted that William, 42, has been supporting his wife. “They are an incredibly strong team, and they will get through this. Their children are their absolute focus,” they said.

An aide added: “William is a human being like everyone else, and it’s been a really difficult time for him, too. I think what’s important to him is that he’s there to support his wife and his children while continuing with his public duties.”

As for when Middleton eventually returns to her royal duties full time, aides said that she’ll “pick up where she left off.”

A friend also said that she’s not stressed about becoming queen one day. “It doesn’t worry her or keep her awake at night. She just wants her kids to be a bit older before she has to step up to the big job. The fact is she was born for the job. It’s almost ingrained in her,” they said.

Since Middleton was diagnosed with cancer, William has stepped up in his royal duties and has been out and about mostly without his wife. He notably attended ceremonies for the anniversary of D-Day in France earlier this month with his father and stepmother Queen Camilla.

Middleton, meanwhile, gave a big update on her health before Trooping the Colour. She said she’s “not out of the woods yet,” and sometimes feels exhausted and has to take recovery day by day.

“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,” she said.

The palace still has not confirmed what type of cancer Middleton has.