The Duke of Edinburgh is expected to spend a third night in hospital for “observation and rest” after being admitted earlier this week after feeling unwell.
Philip was described as being in “good spirits” after he walked unaided into King Edward VII’s hospital in central London on Tuesday evening, on the advice of his doctor.
Buckingham Palace has said admitting the 99-year-old duke was a “precautionary measure”.
There were no reports of visitors arriving at the private hospital on Thursday, but the duke is known for his “no fuss” attitude.
Royal author Penny Junor joked that medics probably “would not want him” on their shift due to his aversion to people “fussing over him”.
The royal family have been working as usual with the Queen carrying out her normal duties of state this week and on Wednesday the Prince of Wales and Duchess of Cornwall visited a Birmingham hospital to learn about vaccine trials.
Ms Junor told BBC Breakfast: “I think he can be quite blunt and I think if he felt people were fussing over him he could be quite outspoken about that.
“This is a man who doesn’t want any fuss made of his 100th birthday, so the fact he’s in hospital and getting some fuss made of him will really irritate him.”
Philip’s hospital admission is front page news and outside King Edward VII’s photographers, cameramen and journalists have gathered to cover any developments.
The duke, who turns 100 on June 10, has so far spent two nights in hospital for an undisclosed reason, although it is not coronavirus-related.
It is understood a doctor was called after Philip felt unwell for a short period and he was taken to hospital by car, where he walked in unaided as a non-emergency admission.
The duke has been spending the latest lockdown with the Queen at Windsor Castle and in January they both received Covid vaccinations.
Buckingham Palace said in a statement issued on Wednesday: “His Royal Highness the Duke of Edinburgh was admitted to King Edward VII’s Hospital in London on Tuesday evening.
“The duke’s admission is a precautionary measure, on the advice of His Royal Highness’s doctor, after feeling unwell.
“The duke is expected to remain in hospital for a few days of observation and rest.”
It is understood the decision to admit Philip was taken with an “abundance” of caution.
The duke last had a spell in hospital when he spent four nights at the King Edward VII’s in December 2019, where he was treated for a “pre-existing condition” and discharged on Christmas Eve.