Prince William thanks public for messages about King Charles and wife Kate

As William took centre stage for the royals, his younger brother Prince Harry, who has become estranged from his family, was set to return to the United States after a flying visit to see their father.

"I'd like to take this opportunity to say thank you ... for the kind messages of support for Catherine and for my father, especially in recent days. It means a great deal to us all." said William, 41, the Prince of Wales.

"It's fair to say the past few weeks have had a rather 'medical' focus. So I thought I'd come to an air ambulance function to get away from it all," he told a gala dinner for London's Air Ambulance Charity where guests included Hollywood actor Tom Cruise.

The event was the heir to the throne's second official public appearance on Wednesday on his return to work having postponed all engagements to look after his three children after Kate, 42, underwent planned abdominal surgery on Jan. 16, and then spent two weeks in hospital recovering.

Since then, his father has undergone treatment at the same hospital for an enlarged prostate, before Buckingham Palace announced on Monday that subsequent tests on the 75-year-old monarch had revealed he had a form of cancer.

With the king postponing public duties to undergo out-patient treatment and Kate not expected to return to engagements until after Easter, the onus is on the remaining royals especially William and Charles' wife Queen Camilla, to provide the public face of the monarchy.

Royal author Robert Hardman said William had already taken on substantial state duties towards the end of Queen Elizabeth's reign when she was hampered by mobility issues.

'QUASI HEAD OF STATE'

"In that regard, it's not that different but obviously there's the burden of expectation," Hardman told Reuters. "On many occasions he will have to stand in, he'll be sort of quasi head of state in much the same way that Prince Charles was when the queen was infirm."

On Tuesday, the king travelled with Camilla to Sandringham House, his home in eastern England, after a brief meeting of about 30 minutes with son Harry who had just flown in to see his father after the king told him he had cancer.

Harry, 39, has barely been on speaking terms with many of the Windsors following his criticism of the monarchy since stepping down from royal duties almost four years ago.

A royal source said there had been no plans for him to see his elder brother William during his visit to Britain. After only about 24 hours in Britain, Harry was seen at Heathrow Airport from where he was expected to fly home to California where he now lives with wife Meghan and their two children.

Despite the diagnosis, Charles is planning to continue with much of his private work as monarch and dealing with state papers. He was due to speak with Prime Minister Rishi Sunak by phone on Wednesday, Sunak's spokesman said.

Buckingham Palace has not given any details of the condition other than to say it was not prostate cancer, but said the king was remaining "wholly positive" and looking forward to returning to public duty as soon as possible.