The King will stress the importance of extending “the hand of friendship… especially in a time of need” in his first public comments since the Princess of Wales revealed her cancer diagnosis.
The pre-recorded audio will be broadcast in his absence at a Royal Maundy service in Worcester Cathedral on Thursday ahead of the Easter weekend.
The King, who announced in February he was undergoing cancer treatment, will say how Jesus set an “example of how we should serve and care for each other”, and how as a nation “we need and benefit greatly from those who extend the hand of friendship to us, especially in a time of need”.
The 75-year-old, who acceded to the throne 18 months ago, will also reamplify his Coronation pledge “not to be served but to serve”.
While the King does not directly refer to his and his daughter-in-law’s health, his words will be interpreted as reflecting on the nation’s response to his and Kate’s challenges as they continue cancer treatment.
Kate, 42, revealed in a video message to the nation on Friday that she had been diagnosed with cancer and was receiving preventative chemotherapy.
She said planned abdominal surgery in January was successful and it was initially thought her condition was non-cancerous.
But tests after the operation found cancer had been present, she said, adding: “This of course came as a huge shock, and William and I have been doing everything we can to process and manage this privately for the sake of our young family.”
She added it has “taken time” to explain her diagnosis to her children George, Charlotte and Louis in a way that is “appropriate for them, and to reassure them that I am going to be OK”.
Kate’s voice broke with emotion at times during her video message which received a global outpouring of support, including from the King, the Duke and Duchess of Sussex, political figures and members of the public.
Click to subscribe to the Sky News Daily wherever you get your podcasts
Her statement followed weeks of global social media speculation and conspiracy theories about her whereabouts after she had not been seen on official engagements since last December.
Meanwhile, the King has stepped back from large-scale public duties while receiving outpatient treatment and the Queen is deputising for him at the ancient Royal Maundy ceremony on Thursday.
Read more:
Harry and Meghan share message of support for Kate
When will Kate return to the public eye?
What is preventative chemotherapy?
However, he is set to attend church on Easter Sunday with Camilla – his most significant public appearance since his diagnosis.
But there will be a reduced number of royals present to avoid the health risks associated with large crowds.
The King recorded the message in mid-March at his desk in Buckingham Palace’s 18th century room.