Britain's King Charles III attended an Easter church service on Sunday, in his most high-profile public appearance since revealing a cancer diagnosis earlier this year.

Charles and his wife, Queen Camilla, arrived by car for the hour-long late morning service at St George's Chapel within the grounds of Windsor Castle, west of London.

The 75-year-old, dressed in a suit and light blue tie, smiled and waved to crowds -- who had queued up for hours for a glimpse of the ailing monarch -- before he entered the chapel.

Leaving the service, Charles and Camilla greeted some of the well-wishers lined up outside, shaking hands and briefly chatting to some in the crowds.

Attending the annual religious ceremony is a long-standing annual tradition for the royals, with other family members in attendance Sunday. 

But heir-to-the-throne Prince William and his wife Catherine were notable absentees this year, as the 42-year-old Princess of Wales receives treatment following her own cancer diagnosis which was revealed just last week.

The situation represents an unprecedented crisis in modern times for the monarchy, as two of its most senior members simultaneously fight serious illness.

Charles was just 17 months into his long-awaited reign when Buckingham Palace announced in early February that he had cancer and would be pausing all public-facing engagements as he commenced treatment.

He had been admitted for surgery for a benign prostate condition in January but was subsequently diagnosed with an unrelated cancer. The type of cancer has not been disclosed.

The king has been continuing with behind-the-scenes work and holding some in-person meetings and increasingly attending official events.

This week he released a pre-recorded Easter audio message -- broadcast in his absence at a Maundy Thursday service in Worcester Cathedral, central England -- stressing the importance of acts of friendship.

The monarch was also photographed receiving community and faith leaders at Buckingham Palace in central London on Tuesday.

Meanwhile last week, photos showed him meeting diplomats at the palace, as well as veterans of the Korean War, at two separate engagements.

Charles said in a statement earlier this month that he would continue to serve "to the best of my ability", and he had been "deeply touched" by people's wishes for his health.

He praised Kate, as she is widely known, for her bravery last week by revealing in a video message that she was undergoing chemotherapy.

She was last seen at a public engagement on 25 December, then underwent abdominal surgery in January, after which cancer was found.

Her Kensington Palace office said at the time that she was not expected to be ready to return to public duties until after Easter.