“I will honor his memory by supporting my father, the king, in any way I can,” he added.
Charles, 73, was officially proclaimed Britain’s new monarch at a ceremony on Saturday, immediately succeeding his mother when the 96-year-old queen died on Thursday.
William, 40, who is now heir to the throne, said the queen “was by my side in my happiest moments. And she was by my side in the saddest days of my life.”
“I knew this day would come, but it will be some time before the reality of life without ‘grandma’ really hits home,” he said.
On Friday, Charles awarded William the title of Prince of Wales, which the current monarch previously held.
William’s wife, Catherine, received the title of Princess of Wales, previously held by Charles’s first wife, Diana.