Prince Philip is facing an operation under general anaesthetic later after being admitted to hospital because of abdominal pains.
The Duke of Edinburgh is set to remain at a Harley Street clinic for around two weeks, which means he will spend his 92nd birthday in hospital.
"The older you get, the longer it takes to recuperate, and he is going under a general anaesthetic which makes it slightly more serious than was originally believed," royal commentator Dickie Arbiter told Sky News.
"Anybody reaching his age starts to slow down. The Duke is robust, he does like working, he does like being alongside the Queen when she is out on an engagement, but he is just going to have to slow down.
"He is going to have to listen to his advisers, and above all he is going to have to listen to his doctors and he is going to have to do what they tell him."
The Queen continued with her engagements for the day, which included officially opening the BBC's rebuilt Broadcasting House in central London.
And as happened last year during the diamond jubilee celebrations, she looks likely to attend several key forthcoming engagements this summer on her own, including the Trooping the Colour next weekend, marking the Queen's official birthday, as well as the annual Order of the Garter ceremony at Windsor Castle and Royal Ascot.
The Duke will certainly miss the Lord's Taverners cricket match in Windsor on Sunday.
The Queen was given a tour of the BBC's new headquarters and declared the building open on Radio 4's Today programme.
In her speech, she recalled visiting the site with the Duke just before her coronation, and made no reference to her husband's health.
Asked how he was doing during her walkabout, she replied: "I have no idea. I mean, he's only just gone in."
Buckingham Palace said the Duke would be having an "exploratory operation" and stressed the hospital visit had been planned, adding that he was in "very good health".
He was seen by doctors Friday morning after spending his first night in hospital, and was described as being "well in himself" upon his arrival on Thursday evening.
Prince Philip was driven to The London Clinic shortly after attending a garden party at Buckingham Palace, where he was seen to be in good spirits while speaking to guests.
The Lord-Lieutenant of Nottinghamshire, Sir John Peace, was among those attending and said Prince Philip had been "telling jokes".
The Duke missed an engagement at the palace on Monday night because he was feeling "under the weather" and had lost his voice, but officials said this was unrelated to his visit to hospital.
The latest operation follows checks by doctors in recent days.
The prince was admitted to hospital twice last year because of a bladder infection that forced him to miss the last of the Queen's diamond jubilee celebrations.
He also suffered a blocked coronary artery the previous Christmas that required surgery.
"This time around there is no clear indication of what the Duke is suffering from. The Queen is understood to be naturally concerned, but is being kept abreast of developments," said Sky's royal correspondent Paul Harrison.
Despite being admitted to hospital on several occasions in recent years, the prince, known for his active lifestyle, has maintained a busy schedule, and in April visited Canada for a "private working visit".
He was at the Queen's side at Westminster Abbey on Tuesday for the 60th anniversary of her coronation and on Wednesday toured a Victorian steamship, the SS Robin, in London's East End.
On Thursday, ahead of the garden party, the Queen invested Prince Philip with New Zealand's highest honour, making him an additional member of the Order of New Zealand to mark the Diamond Jubilee.
Prime Minister David Cameron expressed his best wishes for the Duke on Twitter, writing: "I hope he has a swift recovery."