The Duke of Sussex received a warm welcome from the public during his engagement in London on Wednesday.
The Duke of Sussex is in the U.K. this week but will not see his father.
The event marks the king's first formal public engagement since Feb. 6.
The palace previously announced the king's cancer diagnosis on Feb. 5.
The royal appearance comes a little over a week after the Princess of Wales revealed she'd been diagnosed with cancer.
The Princess of Wales announced last week that she had received a cancer diagnosis.
It is not immediately known which other members of the royal family will attend Sunday's service, but the Wales family is not expected to be there.
Buckingham Palace previously announced on Feb. 5 that King Charles had been diagnosed with cancer.
The Princess of Wales said cancer was detected after her surgery earlier this year.
A spokesperson for the monarch said they have been in the "closest contact" in recent weeks.
