Britain's King Charles III attended a church service at his country residence of Sandringham on Sunday as he continues to receive treatment for an unspecified cancer.

The 75-year-old monarch waved to members of the public before being greeted by priest Paul Williams.

The king announced on February 5 that he had been diagnosed with cancer, and left his London Buckingham Palace home shortly afterwards to undergo treatment at Sandringham.

The diagnosis was made after a "separate issue of concern was noted" and investigated while he was being treated for a benign enlarged prostate.

Charles has taken an indefinite break from royal public duties but continues to perform administrative tasks behind closed doors.

His diagnosis came just 17 months into his reign following the death of his 96-year-old mother, Queen Elizabeth II, on September 8, 2022. (AFP)