Royal Biographer Angela Levin has expressed her concern after The King's hospital stay was extended as he recovers from surgery for an enlarged prostate.
The King, 75, planned on staying at The London Clinic for two nights following the operation on Friday, but has now stayed for a third night.
Speaking on GB News Breakfast, Levin said: "I am worried because you always think that they say a little bit longer than it was going to be so people wouldn't get worried.
"So if it's three nights then you you do get worried. And I thought that Camilla was smiling the other days, but yesterday she seemed not so smiley.
Angela Levin said she is 'concerned'
GB News
"That might have been because of the time the photograph was taken. I think when you are 75, it's very difficult to know actually how one thing can't trigger something else.
"Perhaps he's very tired. Perhaps he's worked so hard for so long that the body is a bit tired of when you've had some sort of operation, then you need longer to come back again."
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GB News host Eamonn Holmes then asked the royal expert why it is not "protocol" for Queen Camilla to visit her husband.
Levin explained: "I suppose it's to avoid lots of people outside making a big panic and all the photographers. Otherwise, I think it's very strange. It's sort of not showing your emotion
King Charles has been recovering in hospital
GB News
She went on to explain that The Queen had however visited her husband on more than one occasion.
She said: "Camilla will show her emotion because she loves him so much, but any normal person would go and visit their loved ones in hospital.
"Royal Family never usually went with them even when they went in, and only rarely would they go visit. Maybe once if it's quite a long stay, but otherwise they don't.
"They must talk on the phone. But I think with this you want to hug, you want to chat, you want all those things. And I think good for her."
Queen Camilla has visited The King this week
ReutersQueen Camilla visited the King on Sunday evening, and was seen smiling in the back of the car when she departed the London Clinic at 7:06pm.
It was the Queen's fourth visit to see her husband since he was admitted to hospital.
Camilla previously visited her husband that day, four hours earlier. King Charles will have a "period of recuperation", according to reports, but will be able to attend to government papers once he is discharged.
Queen Camilla was also seen smiling as she visited her husband at midday. Wearing a teal-coloured top, she smiled as she entered the hospital's back entrance.
Camilla told people outside the London Clinic on Friday that the King was "doing well" after the procedure.