PRINCE William and Queen Camilla will share the "weight" of King Charles' duties during his cancer treatment.
The King, 75, has been advised by doctors to postpone public-facing duties while he receives care for an unspecified form of cancer.
All other working members of the Royal Family are understood to be continuing with public engagements in his absence.
And the Queen, 76, and the Prince of Wales, 41, are set to undertake the brunt of the King's royal duties.
The Sun understands Charles and William have become incredibly close in recent months.
The King has been regularly staying at Windsor Castle on Sunday nights to spend time with his eldest son and heir to the throne.
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What we know so far:
- The King has been diagnosed with a form of cancer
- Prince Harry is flying to the UK to support his father
- Meghan will remain in California with their children
- The King personally called his siblings and children to tell them
- He is receiving expert care from specialist doctors
- His Majesty returned from Sandringham to London to begin treatment today
- Doctors advised the King to postpone public-facing Royal duties
- Camilla is preparing to play an important role supporting the King
- Charles will continue to receive Red Boxes and State documents during his treatment
- The King will continue to meet the Prime Minister for a weekly audience
Meanwhile, William is set to return to public duties following a break to care for his wife Kate and their children after she underwent abdominal surgery last month.
Wills had cancelled his diary for three-and-a-half weeks, but will hold an investiture at Windsor Castle on Wednesday.
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Later the same day, he will attend the London Air Ambulance fundraising gala in London's Raffles Hotel.
The Princess of Wales is unlikely to be seen carrying out any frontline duties until after Easter, sources say.
Prince Harry, meanwhile, will fly to the UK today to support his father.
The Duke of Sussex, who has spoken with King Charles, is believed to have arrived in Los Angeles International Airport in the early hours of this morning.
He is due to board a flight which will land in the UK around midday today.
Meghan, 42, is said to be remaining at their Montecito mansion with children Archie, 4 and two-year-old Lilibet.
The King informed his siblings and children of his diagnosis personally.
Buckingham Palace released a statement at 6pm on Monday evening announcing the King was undergoing treatment for cancer.
It came the day before the late Queen’s Accession Day, marking the moment she took the throne on February 6 1952 following the death of her father, King George VI.
The statement read: “During The King’s recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted.
"Subsequent diagnostic tests have identified a form of cancer. His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties.
"Throughout this period, His Majesty will continue to undertake state business and paperwork as usual.
ROYAL OPERATIONS
How the King and Kate's respective health battles unfolded:
Tuesday, January 16: Kate, 42, is secretly admitted to the London Clinic and undergoes abdominal surgery.
Wednesday, January 17: Kensington Palace announces the princess' operation and says she will remain in the private hospital for 10-14 days.
Over an hour later, Buckingham Palace announces the King, 75, is to have treatment for a benign enlarged prostate and will be admitted to hospital in a few days.
Thursday, January 18: Prince William spends time at his wife's bedside, driving himself away from the back entrance during the low-key, private visit.
The Queen says the King is "fine" and "looking forward to getting back to work" during a visit to the Aberdeen Art Gallery
Friday, January 19: The King flies back from Scotland with the Queen and heads to his Sandringham estate in Norfolk to rest ahead of the procedure.
Monday, January 22: The Queen tells 86-year-old well-wisher Jessie Jackson that the King is "fine", and thanks her for asking, while she carries out engagements in Swindon.
Camilla urges the King to take it easy. An insider told The Sun: "The Queen has told him he needs to slow down a bit."
Thursday, January 25: The King arrives back in London from Sandringham ready for his treatment.
Friday, January 26: The King is admitted to the London Clinic for treatment for an enlarged prostate and also visits the Princess of Wales, who is recovering in the same hospital.
Sunday, January 28: Queen Camilla arrives for her fourth visit on Charles' third day in hospital.
Monday, January 29: King Charles leaves hospital.
Sunday, February 4: Charles seen waving as he walks to church at Sandringham.
Monday, February 5: Buckingham Palace announce King Charles has been diagnosed with cancer.
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure.
"He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible.
"His Majesty has chosen to share his diagnosis to prevent speculation and in the hope it may assist public understanding for all those around the world affected by cancer."
The King will step back from public engagements while he undergoes treatment, but will continue to receive Red Boxes and State documents and hold a weekly audience with the Prime Minister.
The announcement came just eight days after Charles was released from hospital following a three-night stay to undergo a "corrective procedure" for an enlarged prostate.
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Camilla made three visits to the hospital during his stay, revealing he was "fine" and "looking forward to getting back to work" during an official visit to an art gallery in Aberdeen.
Camilla had previously urged workaholic Charles to slow down before he was hospitalised last month.