Catherine, Princess of Wales, won plaudits Saturday for a highly personal video announcing her shock cancer diagnosis, which came just weeks after King Charles III revealed he too is battling the disease.

The candid disclosure, released on Friday evening, leaves the British monarchy in an unprecedented crisis in modern times with two of its most senior members simultaneously fighting serious illness.

Head of state Charles -- 17 months into his reign when Buckingham Palace announced in February that he had cancer and would be cancelling all public engagements -- led tributes to his "beloved daughter-in-law".

The ailing 75-year-old monarch spoke of his pride in "her courage in speaking as she did," shortly after Kensington Palace posted the video on social media.

Following other warm words from UK Prime Minister Rishi Sunak and the White House, British newspapers directly quoted Kate on their Saturday front covers while also praising her "courage" on the inside pages.

"Kate, you are not alone" read the front of The Sun tabloid.

"Dignified Kate has the nation's sympathy," wrote The Standard's columnist Rachel Johnson, sister of former prime minister Boris Johnson.

'Privacy' demand

In her statement Kate, as the 42-year-old is widely known, admitted the diagnosis was a "huge shock" and asked for "time, space and privacy" as she completes chemotherapy for her unspecified cancer. 

In the video -- recorded Wednesday in Windsor, west of London, where the future queen and king live with their three young children -- she insisted she was "well and getting stronger every day".

She said it had taken them time to explain the situation to Prince George, aged 10, Princess Charlotte, eight, and five-year-old Prince Louis, "and to reassure them that I am going to be OK".

"William and I have been doing everything we can to process and manage this privately for the sake of our young family," Kate added.

Commentators commended its frank nature, with the princess speaking directly to the camera while sitting on a garden bench.

"So many people will have been so moved by the way that she conducted herself during that two minute plus broadcast," royal expert Richard Fitzwilliams told AFP.

"But there's no doubt at all that it's a very, very difficult time for the institution of monarchy," he added, noting it was "almost impossible" to plan anything currently involving Charles or Kate.

Royal health woes

Buckingham Palace announced on February 5 that tests had identified Charles had "a form of cancer", without giving further details.  

The discovery was made as Charles underwent surgery for a benign enlarged prostate.  

He has cancelled all public engagements except audiences with the prime minister and ambassadors, and worked on official papers while receiving treatment. 

He has been photographed several times since then, and seen attending church. 

William, Kate and their children are seen as the modern face of the British royal family, and key to its future as it faces declining support among younger people and increasing republican sentiment.

She was last seen at a public engagement on December 25, when she joined the king and other senior royals at a Christmas Day church service.

Kensington Palace announced on January 17 that she was facing up to two weeks in hospital and several months' recuperation following abdominal surgery. 

She was not expected to be ready to return to public duties until after Easter on March 31, a statement at the time said. 

Royal officials did not disclose the exact nature of her condition but said it was not cancer-related. 

'Doing their best'

Friday's announcement abruptly upended those assumptions, with Kate disclosing tests after the operation "found cancer had been present" and that she was now undergoing "preventative chemotherapy".

Kensington Palace said she would return to official duties "when she is cleared to do so by her medical team".

"Preventive chemotherapy after surgery is given to reduce the risk of the cancer coming back in the future," Andrew Beggs, a senior clinical fellow and consultant colorectal surgeon at the University of Birmingham said.

He added it was "a bit like mopping a floor with bleach when you've spilt something on it", noting chemotherapy "kills any spilt cells".

People outside Buckingham Palace on Friday spoke of their shock at the news, despite the weeks of speculation around Kate's health.

"I think they're really doing their best to balance it," said American tourist Hannah Dickerson, 20.

"That's them juggling how to balance telling the public but also privately making sure they do it in their own terms."