Prince William has thanked well-wishers for their supportive messages as twin health alarms embroil the royal family.

It was the Prince of Wales’ first public comments since his father, the King, revealed earlier this week he would be sidelined indefinitely for cancer treatment.

William, who has taken several weeks out of the public spotlight following the Princess of Wales’ abdominal surgery, returned to duty with two outings on Wednesday (British time).

“I’d like to take this opportunity to say thank you, also, for the kind messages of support for Catherine and for my father, especially, in recent days,” he told the crowd at a glitzy formal dinner for the London Air Ambulance charity.

“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.”

William, who previously worked as an air ambulance pilot, rubbed shoulders with another famous aviator at the dinner – Hollywood legend Tom Cruise.

It marked his return as the leading member of the royal family as the King receives treatment for an undisclosed type of cancer.

Catherine is also out of action until after Easter following her major surgery in January. She is recuperating at the Wales’ home on the Windsor Castle estate.

William’s return on Wednesday came after three weeks off to support his family – including children Prince George, Princess Charlotte and Prince Louis – as his wife recovers at home in Windsor.

Earlier, his first engagement, was near home. He presided over an investiture ceremony at Windsor Castle.

Those honoured included former England footballer Ellen White, who was made an MBE. She offered William her sympathies for the twin health emergencies.

“I sent my best wishes to the King,” she said afterwards

“I just think, you know, it must be a difficult time for the family. He (William) was really lovely.”

Also on Wednesday (British time), the King issued his first public statement since revealing his illness.

“I can only say how sorry I am that I cannot be with you in person to mark this momentous milestone,” he wrote in a message to the mark 50 years of independence for the Caribbean island of Grenada.

Tweet from @RoyalFamily

Harry heads home

William’s day of outings also came as his estranged younger brother, Prince Harry, caught a flight back to California after a lightning visit to see the King.

Harry had been told, like other senior royals, personally by the King about his diagnosis, and quickly flew in. He spent about 45 minutes with the King on Tuesday.

Less than an hour later, the King and Queen were seen heading for Buckingham Palace to catch a helicopter to return to their Sandringham estate.

Harry, meanwhile, was later spotted in a VIP area at Heathrow airport, waiting for a flight back to the US. He was in Britain for only about 24 hours.

Tweet from @HelenWade

There was no meeting between him and heir-to-the-throne William, suggesting the King’s illness has not brought any easing of the brothers’ strained relationship.

The King has postponed public-facing duties during his cancer treatment. He will continue with behind-the-scenes work on matters of state.

Downing Street has taken the unusual step of confirming details about Prime Minister Rishi Sunak’s weekly audience with the King, following an agreement with Buckingham Palace. It is expected to continue, but by telephone.

William may step in to represent his father at an event, although none are yet scheduled.

The prince last carried out a major royal event more than three weeks ago, when he travelled to Leeds to make former Leeds Rhinos team-mates Rob Burrow and Kevin Sinfield each a CBE for raising money and awareness for motor neurone disease.

-with AAP