The Duke and Duchess of Sussex have said they wish "health and healing" for Catherine, Princess of Wales, after she shared her cancer diagnosis.
In a brief statement, Prince Harry and Meghan said they hoped Catherine and her family could heal "privately and in peace".
Catherine said on Friday the news was a "huge shock" after an "incredibly tough couple of months".
She said she was "well" and "getting stronger every day".
Details of the cancer have not been disclosed. Kensington Palace says the princess is confident that she will make a full recovery.
Catherine had abdominal surgery in January, but it was not known then there was any cancer.
Subsequent tests revealed cancer had been present.
Harry and Meghan stepped down as senior royals in January 2020.
They moved to California that June, saying they wanted space to raise their son, Archie. A second child, Lilibet, was born the following year.
King Charles has also recently been diagnosed with cancer, and Harry flew to London in early February to see his father a day after he began treatment. He did not meet his brother during the trip.
In an interview afterwards with ABC News, Harry said he was "grateful" for the brief time with his father in London.
He also agreed families could be brought closer by such health concerns.
On Friday, US President Joe Biden's press secretary, Karine Jean-Pierre, said Catherine's news was "terrible" and the White House wished the British royal a full recovery.
"I think it's important that we respect their privacy, especially at this time," she said. "I'm not going to go further than that."
Since Harry announced he was stepping back from royal duties, his relationship with his brother has reportedly soured.
In his memoir, Spare, Harry wrote about feuding with William.
The brothers have rarely been spotted in each other's company in recent years.
But in 2022, William and Catherine and Harry and Meghan unexpectedly appeared together to greet crowds outside Windsor Castle following the death of the Queen.
Earlier this month both brothers took part in a London awards ceremony in honour of their late mother Princess Diana, but did so an ocean apart.
William gave a speech in person at the Diana Legacy Award, while Harry addressed the event by video link from California, after his brother had gone.
