King Charles, Kate Middleton both battling cancer puts focus on Prince William

The double cancer diagnoses of King Charles III and Kate, the Princess of Wales, mark a rare moment for Britain's royal family.

The family's late matriarch, Queen Elizabeth II, lived to the age of 96 with relatively few medical concerns and served for an astounding 70 years, making history as Britain's longest-reigning monarch.

In February, less than two years after inheriting the throne from his mother, it was announced that Charles, 75, had an undisclosed type of cancer for which he had begun treatment, forcing him to step back from his public duties.

At the same time, Kate, the 42-year-old wife of Charles' son Prince William, was fielding speculation from the public over her absence from the spotlight, after undergoing abdominal surgery in January.

For a brief period of time, Kate and Charles were both being treated at the same hospital, the London Clinic, where Kate was recovering from surgery and Charles was undergoing treatment for an enlarged prostate, prior to his cancer diagnosis.

On Friday, the world learned from Kate herself that she had also been diagnosed with cancer after her abdominal surgery.

Kate, who did not reveal the type of cancer she was diagnosed with, began preventative chemotherapy treatments in late February, according to Kensington Palace.

Charles began a "schedule of regular treatments" for cancer on Feb. 5, according to Buckingham Palace, which has not revealed what type of treatment the king is undergoing.

"We have a situation where we have both the king and the Princess of Wales [diagnosed with] some undisclosed forms of cancer," said ABC News royal contributor Victoria Murphy. "It really is unprecedented and it does have that very destabilizing effect."

Diagnoses put 'a lot of pressure' on William

In the background, supporting both his wife and his father, is William, the 41-year-old heir to the throne.

William took some time off from public duties earlier this year to help support Kate and their three young children, Princes George and Louis and Princess Charlotte.

When he returned to work on Feb. 7, William thanked the public for their support of his father and wife, telling attendees at a London gala, "I'd like to take this opportunity to say thank you, also, for the kind messages of support for Catherine and for my father, especially, in recent days. It means a great deal to us all."

Since then, William has publicly kept up the "keep calm and carry on" mentality of the royal family, maintaining a schedule of public engagements, while privately supporting two family members battling cancer.

William did so as rumors swirled online about Kate's health and the state of the couple's nearly 13-year marriage as she took time off from her public duties. Neither William nor Kate responded to the rumors, which grew into a frenzy after William missed a royal engagement in late February due to a "personal matter" and again after the couple released a photo of Kate and their children on Mother's Day in the U.K., which was later retracted by multiple news agencies over editing concerns.

The only response from William to the online frenzy came via a Kensington Palace spokesperson, who pointedly told People magazine of the prince, "His focus is on his work and not on social media."

As he has done privately over the past few weeks, William will now publicly balance his work with his family members' cancer treatments. ABC News understands the Prince of Wales will carry out his engagements when his children return to school after Easter.

With Charles and Kate mostly out of the spotlight to focus on their health issues, William, along with Charles' wife Queen Camilla, are now the most public faces of the royal family.

"With the two star performers, the king and the Princess of Wales, out of action, it does draw a lot of attention on Prince William, who has got a lot to deal with at the moment," said ABC News royal contributor Robert Jobson.

Jobson noted the pressure on William is even greater given the dwindling number of senior royals available for engagements.

"It does, in the bigger picture, put a lot of pressure on Prince William, the Prince of Wales, the heir to the throne, and the other members of the royal family," Jobson said. "And there are fewer and fewer, it seems."

William's only sibling, his younger brother Prince Harry, with whom he has a strained relationship, stepped away from his senior royal role in 2020 along with his wife Meghan, the Duchess of Sussex. The couple currently reside in California with their two children.

With Harry and Meghan gone, the most public-facing senior working royals are down to Charles, Camilla, William and Kate.

Charles' younger sister Princess Anne has also continued to hold royal engagements amid her brother's illness, as has Charles' younger brother Prince Edward and his wife Sophie, the Duchess of Edinburgh.

Neither Kensington Palace nor Buckingham Palace have said when exactly Kate and Charles will resume public duties.