Queen Camilla has revealed the King is "getting on" and “doing his best" following treatment for an enlarged prostate.
Charles, 75, was discharged from the London Clinic on Monday after a three-night stay in hospital.
He smiled and waved as he left the private hospital with his wife Camilla by his side and could be away from public duties for up to a month.
The Queen gave an update as she officially opened a new Maggie’s cancer support centre at the Royal Free Hospital in northwest London on Wednesday.
On arrival, she was introduced to donors including Sir Gerald Ronson, who asked after the King's health.
“He’s getting on, doing his best,” she replied.
When Dori Dana-Haeri, who led the fundraising effort for the new centre, said she was “so pleased” Charles was well, the Queen nodded as she said: “Thank goodness.”
Meanwhile, supporter Sir Michael Pakenham said: "All best wishes to His Majesty for the very best recovery."
Camilla replied: "Thank you very much, that’s very kind. I’ll pass it on."
The Queen's outing comes ahead of World Cancer Day on Sunday 4 February. It marked her 17th visit to a Maggie's centre, a charity of which she has been president since 2008.
Maggie's Royal Free is the 24th centre in the UK providing support for people with cancer, from treatment side effects to financial concerns. The team at each centre also runs groups and activities, all designed to make coping with cancer easier.
Camilla, wearing an emerald green coat, quipped: "I try and get around [the centres] but every time, another one pops up so I never quite catch up."
The King was diagnosed with the benign condition on 17 January while staying at Birkhall in Scotland, after going for a check-up because he was experiencing symptoms.
He is understood to have wanted to share the news to encourage other men to get themselves checked.
Buckingham Palace confirmed the news just 90 minutes after Kensington Palace announced that the Princess of Wales had undergone abdominal surgery.
Charles visited his daughter-in-law at the London Clinic last Friday as he was admitted for his own treatment.
Kate was discharged on Monday after a 13-day stay at the hospital and is now recovering at her home in Windsor. She is not expected to return to public duties until after Easter.
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