British Prime Minister Rishi Sunak said he had been shocked to hear of King Charles' diagnosis of cancer and he hoped the monarch would be able to recover quickly after the illness was caught early.
Buckingham Palace said on Monday that Charles, on the throne for 18 months since the death of his mother Queen Elizabeth, would postpone his public engagements to undergo treatment after the 75-year-old was diagnosed with a form of cancer.
Charles' younger son Prince Harry, estranged from the royal family, is due to arrive in Britain shortly and his elder brother, heir-to-the-throne Prince William, is expected to step up to fulfil some of the monarch's duties.
Sunak told BBC Radio he had been shocked and saddened by the news. "All our thoughts are with him and his family. You know, thankfully, this has been caught early," he said.
Charles is said to be planning to continue with much of his private work as monarch including his weekly audience with the prime minister, and Sunak said he was in regular contact with the king.
"That will of course continue as normal and we'll crack on with everything," he said.
Charles spent the night at home on Monday after beginning out-patient treatment for what the palace has said is a "form of cancer".
It was discovered when Charles spent three nights in hospital last month where he underwent a corrective procedure for a benign enlarged prostate. Beyond confirming it is not prostate cancer, the palace has not given any further details.
The surprise diagnosis dominated Britain's newspapers on Tuesday. "Nation's shock as treatment starts" was one headline in tabloid newspaper The Sun.
It is the latest blow for Charles during his year and a half on the throne.
Early last year, Harry published his autobiography "Spare", which contained damning revelations about his father and elder brother, while Charles has also had to contend with ongoing allegations against his brother Prince Andrew relating to the late sex offender Jeffrey Epstein.
Harry is expected to return to Britain from California where he is living with his American wife Meghan and two children after quitting royal duties in 2020.
The Telegraph newspaper said in a commentary on the announcement that it wished the king well.
"After the upheavals caused by the death of the late Queen and the controversies surrounding the Dukes of Sussex and York (Harry and Andrew), the hopes after the Coronation were for a period of calm and consolidation," it said.
"But illnesses cannot be predicted or avoided, not even by a man who has looked after himself so well. We, along with the rest of the nation, wish His Majesty a speedy recovery."
