“He’s very much trying to maintain a ‘business as usual’ facade,” our royal insider says.
Charles’ medical condition has thrown a royal trip to Australia and New Zealand, planned for late October, into doubt.
“Charles will be keeping everything crossed that his beloved daughter-in-law Kate will recover from her own situation soon, because William will need to get out there and represent the family,” says a source.
In the meantime, Camilla, 76, and the King’s sister Princess Anne, 73, are shouldering most of the load.
Prince Edward and Sophie, the Duke and Duchess of Edinburgh, are currently enjoying a pre-planned break, but are expected to be back on royal duties in early March.
On February 14, Charles left London and returned to Sandringham to recover from his first round of cancer treatment. The King smiled as he waved to onlookers. However, he faces a challenging road ahead as he is expected to regularly return to London for treatment as an outpatient.
“It’s an incredibly difficult time for Charles,” our insider says. “But he’s putting on a brave face and getting on with things.”
King Charles' first statement following his cancer diagnosis, expressed his "most heartfelt thanks" for the good wishes he has received.
"As all those who have been affected by cancer will know, such kind thoughts are the greatest comfort and encouragement," he wrote.
"It is equally heartening to hear how sharing my own diagnosis has helped promote public understanding and shine a light on the work of all those organisations which support cancer patients and their families across the UK and wider world.
"My lifelong admiration for their tireless care and dedication is all the greater as a result of my own personal experience."
King Charles also stepped out at Sandringham, attending church for the first time since the diagnosis.
He was spotted alongside Queen Camilla on February 11, walking to St Mary Magdalene Church.
Prior to this, Queen Camilla had shared an unexpected update on her husband King Charles III, just days after Buckingham Palace revealed the monarch had been diagnosed with cancer on February 6.
Speaking at an event in Salisbury on February 8, she said the King had been doing "extremely well under the circumstances."
"He's very touched by the letters and the messages the public have been sending from everywhere - that's very cheering."
The King will still undertake official paperwork and State business as usual.
He "remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible," the statement revealed.
"His Majesty has chosen to share his diagnosis to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer.
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure."
When the head of state is unable to carry out official duties, the "counsellors of state" can be appointed to stand in. Currently, this includes Queen Camilla, Prince William, the Princess Royal, and Prince Edward.
A Palace spokesperson said, "His Majesty would like to apologise to all those who may be disappointed or inconvenienced as a consequence."