Charles delivered the remarks in a recorded message played at the ancient Maundy Thursday church service at Worcester Cathedral, in central England, which the 75-year-old King missed due to his illness.
"In this country, we are blessed by all the different services that exist for our welfare," he says in the message which was recorded along with a Bible reading earlier this month.
"But over and above these organisations and their selfless staff, we need and benefit greatly from those who extend the hand of friendship to us, especially in a time of need."
The recording was made before his daughter-in-law, Catherine, the Princess of Wales and wife of his eldest son Prince William, issued a video message in which she revealed she was having preventative chemotherapy after cancer was discovered in the aftermath of abdominal surgery in January.
Charles is due to join other members of his family at a scaled-down Easter Sunday service at Windsor Castle, his first public appearance at a royal event since it was disclosed in February that he was suffering from an undisclosed form of cancer.
The custom, which dates back centuries, recognises recipients for their outstanding Christian service from across the country.
"It is for me a great sadness that I cannot be with you all today," Charles says in his message. "This act of worship here in Worcester Cathedral reminds me of the pledge I made at the beginning of the coronation service, to follow Christ's example, not to be served, but to serve.
"That I have always tried to do and continue to do with my whole heart."
