Nine months after he swore to be his father's liege man of life and limb, the Prince of Wales's solemn coronation promise has never been more important.
As the King takes a step back from public-facing duties to undergo treatment for cancer, he will rely on his elder son and heir to represent him and the monarchy when required, just as he did for his mother Elizabeth II.
So as Prince William spends time with his wife and three children over February half-term, he will have much on his mind.
"I think William has certainly realised this and the weight of the world is on his shoulders. He's going to be the King at some stage," royal author Robert Jobson tells HELLO!
"The expectation is on William's shoulders and he must be feeling rather like Her late Majesty Queen Elizabeth when she was 25, suddenly being catapulted into this job which becomes all consuming."
While the King is continuing to carry out his constitutional duties, William may step in to carry out in-person engagements, which the monarch has been advised to postpone during his treatment.
Father and son are in "constant contact", a Palace source tells HELLO! and while they will meet when they can, there will be no fuss or public announcement. They prefer to keep family matters firmly private.
Read the full article in this week's HELLO! magazine.
With the King residing at Sandringham, William, Kate and their children have also reportedly travelled up to their Norfolk home, Anmer Hall, for the school holidays, so they will all be close by.
The Princess of Wales is not expected to return to public duties until after Easter following abdominal surgery and a 13-day hospital stay last month.
The King said: "I would like to express my most heartfelt thanks for the many messages of support and good wishes I have received in recent days.
"As all those who have been affected by cancer will know, such kind thoughts are the greatest comfort and encouragement.
"It is equally heartening to hear how sharing my own diagnosis has helped promote public understanding and shine a light on the work of all those organisations which support cancer patients and their families across the UK and wider world.
"My lifelong admiration for their tireless care and dedication is all the greater as a result of my own personal experience. Charles R."