The palace did not give any details about Charles' cancer but a royal source said that it was not prostate.

The Press Association cited a source saying Prince Harry, who resides in the United States, had spoken with the King about his diagnosis and will travel to the United Kingdom to see him in the coming days.

"The duke did speak with his father about his diagnosis. He will be travelling to UK to see His Majesty in the coming days," the source close to Harry said.

The King has started a schedule of regular treatments but has been advised to postpone his public-facing duties.

He will carry on working behind the scenes on state business and official papers, and returned from Sandringham to London on Monday to commence treatment as an out-patient.

As such, Charles will continue to have meetings with British Prime Minister Rishi Sunak while his wife Queen Camilla will continue with her engagements.

"During The King's recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted," Buckingham Palace said in a statement.

"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 official paperwork as usual.

"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 who are affected by cancer."

Prime Minister Sunak sent his best wishes to the King on X, formerly known as Twitter.

"I have no doubt he'll be back to full strength in no time and I know the whole country will be wishing him well," he said.

Sunak said while he was "shocked and sad" to learn that Charles had cancer, he was relieved the illness had been caught early.

“He’ll just be in our thoughts and our prayers. Many families around the country listening to this will have been touched by the same thing and they know what it means to everyone,” Sunak told BBC radio.

“So we’ll just be willing him on and hopefully we get through this as quickly as possible.”

Asked about the King's diagnosis, US President Joe Biden said: "I am concerned about him", adding that he planned to call Charles later.

"Jill and I join the people of the United Kingdom in praying that His Majesty experiences a swift and full recovery," the president later posted on X.

Also on social media, Australian Prime Minister Anthony Albanese said Charles had always reached out, showing kindness and care for those doing it tough.

"All of us are thinking of him and his family in this very hard time. We wish him well for a speedy recovery," Albanese said.

In a similar post Canadian Prime Minister Justin Trudeau said like others around the world he was thinking of the King.

"We’re sending him our very best wishes – and hoping for a fast and full recovery," Trudeau said.

Charles was discharged from The London Clinic a week ago after undergoing treatment on an enlarged prostate.

Buckingham Palace announced on January 16 that the King was to have treatment for an enlarged prostate but that the condition was benign.

The palace on Monday announced the King's cancer diagnosis at 6pm (5am AEDT on Tuesday) .

Charles, who acceded to the throne just 17 months ago, was last seen on Sunday when he attended church in Sandringham.

with PA

License this article