King Charles Attends Easter Service in First Major Public Appearance Since His Cancer Diagnosis

King Charles made his most significant public appearance on Sunday since beginning cancer treatment, leading the royal family for Easter church service at Windsor Castle.

The King, 75, and Queen Camilla, 76, arrived at St. George's Chapel for the Easter Mattins Service by car. He greeted the church clergy upon his arrival and then waved to the gathering crowd outside the chapel before making his way in.

Following medical advice, the royal couple were seated apart from the rest of the royal family and other attendees during the service to minimize the risk of infection. As per the orders of King Charles's doctors, neither he nor Camilla, who looked festive in a green coat dress accessorized by an emerald and diamond brooch that belonged to the late Queen Elizabeth, will attend a reception or host a private family lunch after the service.

After the service, the King and Queen conducted a surprise walkabout, thanking members of the congregation, estate workers and the general public who gathered outside the church to show their support for the monarch. Charles smiled as he shook hands, responding to one well-wisher's "get well soon" with, "I'm doing my best!"

Attending the service alongside Charles and Camilla were Prince Andrew and his ex-wife Sarah Ferguson, Princess Anne and her husband Vice Admiral Sir Tim Laurence and Prince Edward and his wife Sophie, Countess of Wessex. Several key members of the royal family were absent from the service, notably Kate Middleton, Prince William and their kids — Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5 — who did not attend as they typically would following Princess Kate's public announcement about her cancer diagnosis last week.

Charles' plans to attend the service were announced last week by Buckingham Palace following speculation about whether he would attend amid his cancer treatment. The confirmation came after it emerged that the sovereign hoped to attend church on the holiday, which was expected to be a smaller-scale celebration. Doctors have advised the King to limit the number of people he comes in contact with during his treatment for cancer, a diagnosis that the palace announced on Feb. 5.

The monarch has postponed public-facing duties while continuing to work behind the scenes and hosting small audiences at Buckingham Palace.

Kate, William and their children are spending the holiday weekend privately at home after the Princess of Wales, 42, shared the news about her health last Friday. Princess Kate revealed that she was receiving "preventative chemotherapy" after post-operative tests following her abdominal surgery in January "found cancer had been present" though her condition was initially thought to be non-cancerous. The royal mom said that she and William took time to explain everything to George, Charlotte and Louis and asked for privacy during this time.

"We hope that you will understand that, as a family, we now need some time, space and privacy while I complete my treatment," the Princess of Wales said towards the end of her video message, filmed on March 20 in the garden of Windsor Castle.

Princess Kate kept the type of cancer private, as did King Charles. The diagnoses mark the first time in the monarchy’s history that two senior royals are facing the disease simultaneously.

"He has always had a very good bond with her. I don’t think it is presumptuous to say that she is like the daughter he never had," royal biographer Sally Bedell Smith tells PEOPLE exclusively in this week's cover story. "He shares with William an impulse to protect her. They are in this together, Kate and the King."

King Charles is "progressing well" in his treatment, and there is an "amplified confidence" amid the sophisticated treatment receiving, a royal source tells PEOPLE.

"He is positive, the doctors are optimistic," the source adds.

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The insider says that the sovereign is planning for a "summer diary," but won’t commit to any engagements as they will rely on "medical advice nearer the time." King Charles' recent outings have been capped at small numbers of people, but as summer approaches, more engagements could happen outdoors.

"One thing that has been wholly undiminished is his appetite for work," the royal source says.