With his father King Charles III undergoing cancer treatment and his wife recovering from surgery, Britain's Prince William returned to royal frontline duties on Wednesday.

The king's shock cancer diagnosis, announced on Monday, and Catherine's abdominal operation have left William, 41, shouldering a heavy royal burden.

Charles's eldest son and heir to the throne, postponed public engagements to care for his wife, the Princess of Wales, and their three children after she was admitted to hospital on January 16.

But on Wednesday he hosted an investiture ceremony at Windsor Castle, west of London, handing out honours for citizens reognized for their community work and other good deeds.

The prince last carried out a major royal event more than three weeks ago.

Later, he will attend a London Air Ambulance fundraising gala.

Prime Minister Rishi Sunak made a brief reference to the king's cancer diagnosis in parliament.

"I know the thoughts of the house and the country are with the king, and his family," he told the House of Commons.

"We wish his majesty a speedy recovery and look forward to him resuming his public-facing duties in due course, he added.

William is also expected to take on some of his father's duties while he undergoes treatment, alongisde fellow senior royals Princess Anne and Charles's wife Queen Camilla.

Buckingham Palace has not specified the type of cancer afflicting the 75-year-old monarch, although it is understood not to be prostate cancer.

Sunak said on Tuesday it had been "caught early".

'Very deep' rift -

The diagnosis comes just 17 months into Charles's reign following the death of his 96-year-old mother, Queen Elizabeth II, on September 8, 2022.

Adding to the drama, the king's estranged son Prince Harry flew back from his US home on Tuesday.

The pair had a 45-minute meeting at Charles's Clarence House residence in London.

Charles was seen afterwards leaving for his Sandringham country estate in eastern England.

Harry's return has sparked speculation it could serve as a catalyst to heal family tensions that have blighted Charles's reign.

Harry and his brother William have reportedly not spoken in months because of bad feeling caused by Harry's criticisms of his family.

Harry quit royal duties in 2020 and relocated to California where he now lives with his American wife Meghan and their two young children.

Harry has repeatedly aired his complaints about the way he feels he and his wife were mistreated during their time as working royals, culminating in January 2023 with his autobiography "Spare".

The Press Association news agency reported, however, that there were no plans for the brothers to meet while Harry was in the UK.

Royal commentator Richard Fitzwilliams described the rift between William and Harry, also known as the Duke of Sussex, and the rest of the royal family as "very deep".

Reports said he had stayed at a luxury London hotel overnight following Charles's decision to block him from using his former home on the Windsor estate.

It was not known how long Harry was due to stay in the country.

People expressed sympathy for William, who they noted now faced the double burden of maintaining his family life with extra official duties.

"He's got a hard job because his wife is poorly at the moment, so that's an added pressure on poor William, but I'm sure that he will cope," pensioner Sue Hazell told AFP outside Buckingham Palace on Tuesday.

Kate is expected to be out of action until at least March 31, her office has said.

Officials have not given details of her surgery except to say it was not linked to cancer.

Canadian tourist Sarah Paterson, a 44-year-old entrepreneur, said that William must be "beside himself" given the recent deaths of his grandfather and his grandmother, along with the health problems faced by his father and his wife.

But she was "1,000 percent" confident that William would be a good stand-in, adding: "I think he'll probably be king sooner than he hoped."