Charles, 75, had spent three nights in hospital last month after undergoing a corrective procedure for a benign enlarged prostate, when a separate issue of concern was noted.

The palace said that tests had identified a form of cancer.

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 cancer 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 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.”

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.

– AAP