Prince Harry arrives in London after King Charles III's cancer ...

Prince Harry arrived in London and spent about 45 minutes at his father's royal residence, Clarence House, on Tuesday as King Charles III was pictured for the first time since his cancer diagnosis was made public.

Harry, who has fallen out with the King and other royals in recent years, boarded a flight from Los Angeles shortly after the monarch's condition was announced by Buckingham Palace — although he was already aware of it.

The Duke of Sussex landed in Britain on Tuesday afternoon and went straight to Clarence House, where King Charles was staying, about 2:45pm local time.

At 3:30pm, the King and Queen Camilla were photographed waving to onlookers as they left the property on The Mall and were driven to nearby Buckingham Palace.

The couple then left for another royal residence, Sandringham Estate in Norfolk, via helicopter. Harry left Clarence House separately to his father.

Prince Harry's convoy arrives at Clarence House on Tuesday.(AP: Kin Cheung)

Meetings between Harry and the King have been scarce since the 39-year-old stepped back from royal duties almost four years ago.

The Duke of Sussex attended his father's coronation last May, but it's understood there are no plans for him to reunite with his brother, Prince William, while he's in the UK.

Charles smiled and waved to passers-by as he was driven from Clarence House. It was the first time he'd been seen publicly since the diagnosis was made public.

The palace has said the King remained "wholly positive", and Prime Minister Rishi Sunak earlier said the cancer had been caught early.

Buckingham Palace has not released details about the type or stage of cancer the King has, or a prognosis.

The palace says the 75-year-old monarch will step back from public-facing commitments but continue official duties as the head of state of the UK and Australia.

Harry had been living in the US with his wife Meghan and two children after quitting royal duties in 2020.

The royal helicopter lands at Buckingham Palace.(Reuters: Peter Nicholls)

Earlier on Tuesday, Mr Sunak said he was "shocked and sad" to hear of King Charles's cancer diagnosis.

Mr Sunak told the BBC he was thankful the illness was "caught early", and he hoped the king made a speedy recovery.

"All our thoughts are with him and his family. You know, thankfully, this has been caught early," he told the BBC.

"He'll just be in our thoughts and our prayers — many families around the country listening to this will have been touched by the same thing and they know what it means to everyone.

"So we’ll just be willing him on and hopefully we can get through this as quickly as possible."

Mr Sunak told BBC Radio on Tuesday he was in regular contact with the king and that would continue during his treatment.

"That will of course continue as normal and we'll crack on with everything," Mr Sunak said.

Charles, who became King in September 2022 following the death of his mother Queen Elizabeth, has begun a series of treatments, the palace said, adding he was looking forward to returning full-time to his duties as soon as possible.

The news of the diagnosis comes after Charles spent three nights in hospital last month where he underwent a corrective procedure for a benign enlarged prostate.

The palace said a separate issue of concern had been spotted during that hospital stay, but has not given any further details beyond saying tests had revealed the King had a "form of cancer".

"No further details are being shared at this stage, except to confirm that his majesty does not have prostate cancer," the palace said.

"Throughout this period, his majesty will continue to undertake State business and official paperwork as usual."

Prince William is expected to step up to fulfil some of the monarch's duties.

Earlier, Australian Prime Minister Anthony Albanese said the "thoughts of all Australians are with King Charles and his family".

"Australians know that His Majesty King Charles has always reached out to us in our country's toughest moments, showing kindness and care for those doing it tough," he said on social media.

"All of us are thinking of him and his family in this very hard time.

"We wish him well for a speedy recovery."

Reuters/ ABC