King Charles "is doing extremely well under the circumstances" following the start of his cancer treatment, Queen Camilla has said.
Asked how the King was doing at an event at Salisbury Cathedral on Thursday evening, she said: "He is very touched by all of the letters and messages the public have been sending from everywhere."
"That's very cheering," she added.
Buckingham Palace announced the King's cancer diagnosis on Monday.
It was detected while the monarch was undergoing treatment for an enlarged prostate last month, the Palace said. The type of cancer has not been disclosed, though it has confirmed it is not prostate cancer.
The King has stepped back from all public-facing duties while he undergoes treatment, the Palace said. Senior royals, including the Queen and the Prince of Wales will take on his duties for some events.
On Wednesday, the King was pictured for the first time since his diagnosis was made public alongside the Queen in a car leaving Clarence House in London to catch a helicopter to Sandringham in Norfolk.
Buckingham Palace has said the monarch will continue with paperwork and his constitutional duties during the unspecified treatment.
Before the King left London, his son, the Duke of Sussex, travelled from the US to visit him. He was seen at Heathrow Airport the following day returning to Los Angeles, where he lives with his wife, Meghan, and their two children.
The Queen was speaking on Thursday evening at a musical concert celebrating the work of local charities at Salisbury Cathedral in Wiltshire.
Among those she spoke to was a member of the Wiltshire Air Ambulance team, paramedic Ben Abbott, who said: "I do hope His Majesty is doing well Ma'am, we're all really sad to hear the news."
Aware that the Queen has a house in the village of Lacock, Wiltshire, Mr Abbott added: "I told her we always wave when we fly over Lacock and she said she always waves back."
Other members of the Royal Family have continued with their royal duties too since the announcement concerning the King's health.
On Wednesday the Prince of Wales attended a fundraiser for the London Air Ambulance.
During the event, he expressed his gratitude for the "kind messages" of support for both the King and his wife, the Princess of Wales, who is recovering from abdominal surgery.
Earlier in the day, he carried out an investiture ceremony at Windsor Castle. His return to public duties followed a three-week absence which began after Catherine underwent the surgery.
She is not expected to return to royal duties until after Easter.
The Princess Royal also attended official engagements on Wednesday, but Anne did not respond to questions about the King when asked by reporters while leaving a military barracks in Bicester, Oxfordshire.