Royal watchers have been concerned about King Charles since he revealed his cancer diagnosis last month but there was cause for optimism on Sunday.
After the King and Queen attended the Easter Mattins service and stopped to greet well-wishers, a Buckingham Palace source described the public outing by saying "Today was a significant step" for the royal.
They continued: "As can be seen, The King has responded to treatment very encouragingly over past weeks and his doctors were thus able to adjust their guidance slightly on what His Majesty is now able to undertake, including attendance at the Easter service and greeting well wishers who had kindly turned out to show their support.
"To be clear, His Majesty's treatment continues and caution is of course the watchword, but as diary plans are evolved towards summer, we hope to see more of these carefully-calibrated steps towards the resumption of some public facing duties for The King, with adjustments made where necessary.
"Nothing will be confirmed until nearer the time, plans remain in flux and are subject to medical guidance, but there is great hope and optimism from both doctors and the patient.
"While HM has of course been undertaking all his official duties in private, and has been photographed at a number of smaller Palace engagements, hopefully today has offered wider public reassurance that His Majesty is doing well and that the road ahead is looking very positive."
The update comes after Charles and Camilla were joined by their family members at St. George's Chapel, Windsor Castle for the service.
As planned, the Prince and Princess of Wales were absent, because the Princess is taking time out from public engagements while she undergoes preventive chemotherapy for her own unspecified cancer, which was discovered following major abdominal surgery.
While the King appeared in good spirits at the event, he has slimmed down his calendar since the news of his health was revealed.
Earlier in the week, he skipped the Royal Maundy service in Worcester Cathedral on Thursday, with his wife standing in for him in a historical move. The Royal Maundy service commemorates Jesus's Last Supper when he washed the feet of his disciples as an act of humility the day before Good Friday.
This year, the Queen presented 75 women and 75 men – signifying the King’s age – with two purses, one red and one white, filled with Maundy money.
Charles, meanwhile, released a heartfelt personal message recorded at Buckingham Palace, which was broadcast during the service.
Speaking in his first public address since his "beloved" daughter-in-law Princess Kate revealed her own diagnosis and treatment plan, the monarch emphasised the importance of kindness.
In his address, he talked about how Jesus set an "example of how we should serve and care for each other," and said that as a nation "we need and benefit greatly from those who extend the hand of friendship to us, especially in a time of need".
Charles was photographed recording the message earlier this month, and the picture shows him smiling as he sits at his desk in Buckingham Palace's 18th Century Room.