Former BBC Royal Correspondent Michael Cole says the King is "determined" to attend Sunday's Easter service, as the monarch continues his cancer treatment in private.
In a pre-recorded message to mark Easter weekend, the Monarch stressed the importance of "care and friendship", especially "in a time of need".
King Charles also noted in his first public words since the Princess of Wales also announced her cancer diagnosis how Jesus was an "example of how we should serve and care for each other".
Speaking to GB News, Michael Cole said the "effects of his cancer treatment" are visible in a newly released photograph to mark Easter weekend, commenting on his "less fuller face" and "thinning lips".
Michael Cole says King Charles's cancer treatment is 'really showing its effect'
PA / GB News
Cole noted that in his Maundy Thursday speech today, the King "will not be mentioning his cancer or the Princess of Wales's cancer diagnosis", but instead sending a "clear message about friendship and serving others".
The Princess of Wales announced last Friday that she has begun "preventative" cancer treatment, following her abdominal surgery in January.
Queen Camilla has attended the Royal Maundy service at Worcester Cathedral
PA
Speaking to GB News hosts Ellie Costello and Stephen Dixon, Cole explained that the King's cancer diagnosis and treatment "hasn't been easy" and is "obviously a great strain for anybody".
He said: "While you're bearing the burdens of being head of state at the same time as you're being treated, I'm sure it isn't easy for him.
"It's a very great shame for me to see him like this. I was five years old when he was born, I've watched him and reported on him throughout his life, and I'm very sad to see him in this way."
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Despite the King missing Thursday's service in Worcester, Cole praised Queen Camilla for attending in his absence and said she will be "doing the honours" and "distributing lovely little purses containing silver coins to worthy local people".
When asked if the King will be in attendance at Sunday's Easter Service at St George's Chapel in Windsor, Cole said the monarch is "determined to be there" and his appearance will be "very heartening" for royal fans who are "concerned" for his health.
Cole told GB News: "He's determined to be there with as many members of his family as possible in attendance.
"It's an important day, of course. It's a day of renewal. Tomorrow is the most solemn day of the year, Good Friday, and then rebirth with all that means."
Michael Cole says King Charles is 'determined' to attend Sunday's Easter service
GB News
Michael Cole also noted the King has "other worries" during this time, alongside his cancer treatment, including Princess Kate and Prince Harry.
He said: "It's an important time, and of course he has other worries, not least Prince Harry out in California and what he's doing.
"And other members of the Royal Family in trouble. Real trouble at the moment is notably Kate, his beloved daughter-in-law, as he always refers to her."