Buckingham Palace announced on Tuesday that Britain’s head of state, King Charles III, will attend an Easter Sunday church service, his most high-profile event since revealing his cancer diagnosis.

King Charles and his wife, Queen Camilla, will attend Easter morning prayers at St George’s Chapel at Windsor Castle, a long-standing annual tradition for the British royal family.

It comes after the 75-year-old king’s daughter-in-law, Catherine, Princess of Wales, said last week that she was also undergoing treatment for cancer.

It plunges the monarchy into an unprecedented crisis in modern times, with two of its most senior members battling serious illnesses at the same time.

Catherine, 42, married Prince William, King Charles’ eldest son and heir, making her the future queen.

King Charles has been on the throne for just 17 months when the royal family announced in early February that he had cancer and would suspend all public-facing engagements while he began treatment.

In January, he was admitted to hospital for surgery for a benign prostate condition but was later diagnosed with an unrelated cancer.

The type of cancer has not been disclosed.

He has continued to work behind the scenes, holding some in-person meetings and increasingly attending official events.

On Tuesday, he was pictured receiving community and religious leaders at Buckingham Palace in central London.

Last week, photos showed him meeting with diplomats and Korean War veterans at the palace at two separate events.

King Charles said in a statement earlier this month that he would continue to serve “to the best of my ability” and that he was “deeply touched” by people’s wishes for his health.

He praised Kate’s bravery in a video message last week when he revealed she was undergoing chemotherapy.

She was last seen at a public event on December 25 and then underwent abdominal surgery in January before the cancer was discovered.

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

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