British Prime Minister Rishi Sunak says he is thankful King Charles's ...

British Prime Minister Rishi Sunak says he is "shocked and sad" to hear of King Charles's cancer diagnosis.

Mr Sunak has told the BBC he is thankful the illness was "caught early", and hopes the king makes 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."

Buckingham Palace has not released details about the type or stage of cancer 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.

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 had 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 while Prince Harry is expected to return from California where he is living with his wife Meghan and two children after quitting royal duties in 2020.

Earlier, 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