Royal Author Robert Hardman says King Charles could be at risk if he was to continue public-facing duties, as the monarch begins treatment for cancer.
In a statement released on Monday evening, Buckingham Palace confirmed that the King has been diagnosed with "a form of cancer" and will postpone his duties.
This follows the King's recent hospitalisation for a "benign enlarged prostate", where he underwent a "corrective procedure".
Queen Camilla was by his side as he left The London Clinic last week having previously told concerned onlookers that the King was "fine" and "looking forward to getting back to work".
Robert Hardman says the King will need to be 'kept away' from large crowds
PA / GBN America
On Tuesday, King Charles and Queen Camilla travelled to Sandringham following a meeting with estranged son Prince Harry.
The Duke of Sussex flew to London from Los Angeles overnight on Monday and met with his father for a "maximum of 45 minutes" at Clarence House. A helicopter was then spotted leaving Buckingham Palace's grounds.
Following the Palace's advice for the King to step back from public royal duties, author Robert Hardman said the King's cancer "is not prostate cancer", but it is not known which cancer he is being treated for.
Speaking to GBN America, he revealed the main reason why the King may have been withdrawn from facing the public, as he undergoes cancer treatment in private.
King Charles is to receive treatment after being diagnosed with 'a form of cancer'
PA
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Hardman told host Nana Akua: "He's now undergoing his treatment, he's in good spirits, he's still working, but he's having to cancel his public engagements because he can't be out and about in public.
"His doctors want to keep him away from large crowds and from possible infections."
Hardman added that the King will still continue to take on "desk work" throughout his treatment and recovery, as Prince William and Queen Camilla take on the majority of his duties.
Prince William will undertake the first of the King's engagements since his diagnosis announcement, leading investitures at Windsor Castle.
Robert Hardman says King Charles is 'determined to do as much work as he can'
GBN America
In praise of the King's determination to recover from his cancer, Hardman said Charles is "determined to get on with the job as much as he can" whilst undergoing his "outpatient treatment".
Nana Akua agreed, adding: "He was very wise in changing a little bit of legislation to make sure that he could include others so they could take over if he were incapacitated in any way.
"And he's been very polite to not humiliate Prince Harry or Prince Andrew."