In mid-February, the King was photographed for the second time since Christmas when he arrived back in London after a short break in Sandringham, his wife Queen Camilla loyally by his side.
“Camilla has declared the only reason she would leave him is to shoulder extra royal responsibilities, so he can focus on his health,” a royal insider said at the time.
Still, it was clear the couple seemed anxious as they arrived at Clarence House, where they will base themselves while Charles undergoes treatment.
Just days prior, Charles was also seen in Sandringham where he attended a service at St Mary Magdalene Church.
The monarch hasn’t confirmed as yet what type of cancer he has been diagnosed with, only revealing it was discovered during surgery to treat an enlarged prostate.
Despite this, The Mirror newspaper reports Charles is “keen to be seen” as he confronts his cancer treatment “head on”.
All the King’s public appearances remain cancelled, and the palace has yet to announce when he could be well enough to return to full-time duties, but he insists on continuing his weekly meetings with Prime Minister Rishi Sunak.
“He’s very much trying to maintain a ‘business as usual’ facade,” our royal insider says.
The King is also still undertaking official paperwork and additional State business as per usual.
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.
King Charles' first statement following his cancer diagnosis, expressed his "most heartfelt thanks" for the well wishes he had 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 [the] wider world.
"My lifelong admiration for their tireless care and dedication is all the greater as a result of my own personal experience."
Before 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."
In the statement from Buckingham Palace that first confirmed his cancer diagnosis, it was revealed that Charles remained "wholly positive about his treatment" and was eagerly awaiting the day he could return to "full public duty."
"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 The Queen, The Prince of Wales, The Duke of Sussex, The Duke of York and Princess Beatrice.