King Charles has ‘to accept limitations’ as Palace 'prepares' for William to be monarch

Sally Bedell Smith told People: "Although the King is carrying out his duties with typical determination, he has had to accept limitations during his cancer treatment.

"As a result, William has been taking on more responsibilities, and he and Kate have been preparing for their future roles sooner than they would have expected."

A palace source added: "There's a sense of calm before the storm.

"They are next in line for the biggest job of their lives, and of course, Kate's health has taken priority, but it's also allowed everyone to take a step back and figure out what's important right now."

Princess Catherine, 42, announced she was diagnosed with an undisclosed form of cancer in March and temporarily stepped back from public duties.

In a boost to her health, Kate revealed she had completed her preventative chemotherapy in an emotional video posted to social media in September.

The future queen has since made several major public appearances, including consecutive Remembrance Weekend duties and hosting her fourth Annual Together at Christmas carol concert at Westminster Abbey.

Royal insiders have confirmed that contingency plans for succession are always in place, with a source telling People: "It is something the institution is always mindful of."

The Prince and Princess of Wales's priorities remain unchanged, with their children Prince George, 11, Princess Charlotte, nine and Prince Louis, six, at the heart of everything they do, according to friends of the royals.

One friend of the couple said: "Life-threatening illnesses bring a reconsideration of priorities."

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Speaking on Princess Kate, the friend added: "She and William have always made it clear that family is the most important thing."

Prince William and Princess Kate are set to spend their Christmas at Sandringham with their children and King Charles.

They are expected to attend the Royal Family’s celebrations and participate in the traditional walk to and from St Mary Magdalene Church.

The Wales family are believed to have already travelled to their ten-bedroom Georgian residence, Anmer Hall, situated on the Sandringham estate in Norfolk, following the end of the school term.