King Charles III's estranged son Prince Harry reportedly arrived in London on Tuesday after his father's diagnosis of cancer, which his doctors said was "caught early".

Harry, who now lives in California with his actress wife Meghan and their children, has been at war with his family since quitting royal life and launching a barrage of score-settling criticism in his best-selling autobiography "Spare".

The Telegraph and The Sun newspapers reported that Harry arrived at Heathrow airport following an overnight flight from Los Angeles.

His car was reportedly seen being driven from the airport accompanied by two police cars, less than 24 hours since Buckingham Palace made Charles's cancer diagnosis public.

Royal watchers say that while Charles's ill health may be a catalyst for a rapprochement between father and son, healing the rift with his brother Prince William, heir to the throne, would be more difficult.

Buckingham Palace has not specified the type of cancer afflicting the 75-year-old monarch who will now step back from public-facing royal duties to complete his treatment.

UK Prime Minister Rishi Sunak said the cancer had been "caught early".

Charles is just 17 months into his reign having waited decades to begin the job he was born to do following the death of his 96-year-old mother, Queen Elizabeth II, on September 8, 2022.

Citizens expressed shock and sympathy at the news.

"We were just incredibly sad and hope... that he gets over it," said Sue Hazell, a retiree from the northern English city of Doncaster, as she visited Buckingham Palace.

"It's not nice to hear anyone's been diagnosed with cancer," added Sarah Firisen, 55, a software sales worker.

"I feel kind of bad for him. He waited all these years to be king."

The diagnosis will prolong a frontline shortage of royals created by Charles's admission to hospital last month for a benign prostate procedure and the almost simultaneous hospitalisation of Catherine, Princess of Wales. The royals' health issues have left 76-year-old Queen Camilla as the highest profile royal, with support from Charles's sister Princess Anne, 73.

- 'Wholly positive' -

"Thankfully, this has been caught early and now everyone will be wishing that he gets the treatment that he needs and makes a full recovery," Sunak told the BBC.

"I think that's what we're all hoping and praying for and I am, of course, in regular contact with him and will continue to communicate with him as normal," he said.

Charles has generally enjoyed good health, barring injuries from polo and skiing.

But the palace said that during his recent hospital procedure to treat a prostate enlargement, "a separate issue of concern was noted" and subsequent diagnostic tests had identified "a form of cancer".

"Many families around the country who are listening to this will have been touched by the same thing and they know what it means for everyone," Sunak said.

"So we'll just be willing him on and hopefully we'll get through this as quickly as possible."

Doctors have advised Charles to postpone any engagements, though he will continue to "undertake state business and official paperwork as usual", the palace said.

The king "remains wholly positive" and "looks forward to returning to full public duty as soon as possible", it added.

- Shock announcement -

Charles won plaudits last month for being open about his prostate condition, with doctors saying many more members of the public had subsequently come forward with symptoms.

He was discharged from a London hospital on January 29 after a three-night stay for corrective surgery.

The palace added that the king had chosen to share his cancer diagnosis "to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer".

Messages wishing Charles a swift recovery flooded in.

US President Joe Biden said he was "concerned" about Charles, while Canadian Prime Minister Justin Trudeau said he and the nation were "thinking" of him.

Australian Prime Minister Anthony Albanese told reporters: "All Australians will be sending their best wishes to King Charles for a speedy recovery."

French President Emmanuel Macron also wished Charles a "speedy recovery", adding on X that "Our thoughts are with the British people."

The diagnosis is a blow to the royal family, who has been spread thinly in recent weeks.

Catherine, 42, who is the wife of Charles's son and heir to the throne Prince William, underwent abdominal surgery at the same hospital where Charles was treated.

She left on the same day as Charles last Monday following a stay of around two weeks, and may not return to public duties until late March.

William, 41, also stepped back temporarily from planned engagements to help care for their three children, though he is expected to return to royal duties this week.