The King has begun immediate treatment after doctors discovered cancer during a recent procedure for a benign prostate condition.

Buckingham Palace said on Tuesday morning (AEDT) the cancer was “noted” while the monarch was in hospital recently for treatment on an enlarged prostate.

Media are reporting his latest diagnosis is not prostate cancer. The form of cancer has not been revealed, nor has the King’s prognosis.

Doctors have advised him to step back from “public-facing duties” while he undergoes cancer treatment that will begin on Tuesday.

However, during the treatment period, the King will continue to attend to state business and official paperwork.

It is expected other senior royals will help by standing in for him at public engagements, writes the BBC.

The 75-year-old sovereign spent three nights in hospital last month after undergoing a corrective procedure for an enlarged prostate.

“During the King’s recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted,” the statement from Buckingham Palace said.

“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 Press Association reports that the King’s youngest son, Prince Harry, who lives in the US, has spoken with him about his diagnosis.

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

Tweet from @RoyalFamily

The palace said the King, who is known to eat well and keep fit, was upbeat about his health prognosis and was sharing the news to raise awareness.

“The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure,” it said.

“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.”

The King and Queen at church in Sandringham on Sunday. Photo: Getty

The latest royal health shock comes as the Princess of Wales is recovering after abdominal surgery that will keep her out of the public eye until after Easter.

The King, who acceded to the throne just 17 months ago, was seen at a church service in Sandringham on Sunday, where he waved to crowds.

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

Buckingham Palace announced on January 16 that he would have treatment for the benign condition.

The King’s cancer diagnosis was announced at 6pm Monday (local time).

-with AAP