British head of state King Charles III will make a limited return to public duties next week, after doctors said they were "very encouraged" by the progress of his treatment for cancer.

His first engagement with Queen Camilla would be to a cancer treatment centre on Tuesday. The couple will also host Emperor Naruhito and Empress Masako of Japan for a state visit in June.

The announcement follows a difficult spell for the royal family after both Charles, 75, and his 42-year-old daughter-in-law Catherine, Princess of Wales, revealed that they had cancer.

Catherine, the wife of heir to the throne Prince William, made the shock announcement that she was undergoing preventative chemotherapy just over a month ago in a video posted to social media.

Announcing Charles's limited return, a Buckingham Palace spokesperson said it was "too early to say" how much longer his treatment would continue.

But they noted that his medical team were "very encouraged by the progress made so far and remain positive about the king's continued recovery".

"The pacing of the king's programme will be carefully calibrated as his recovery continues, in close consultation with his medical team," the spokesperson added.

The king and Queen Camilla will host the Japanese royal couple at Buckingham Palace in late June, the palace said, confirming reports in the Japanese media last month.

The Japanese royals both attended the state funeral of Charles's mother, Queen Elizabeth II, in September 2022, in their first overseas trip after the emperor's enthronement.

They also attended the reception the king hosted at Buckingham Palace the evening before the funeral for heads of state and official overseas guests.

An earlier state visit which had been planned for early 2020 had to be postponed because of the coronavirus pandemic. (AFP)