The Duke of Edinburgh, 90, has spent two nights in hospital being treated for a bladder infection, after missing the end of the Diamond Jubilee celebrations.
A Buckingham Palace spokesman said the duke was in “good spirits”.
The duke was admitted to hospital on Monday, just hours before the Jubilee Concert outside Buckingham Palace.
The Queen is attending a lunch with Commonwealth leaders, as she continues to mark 60 years of her reign.
The duke, who had treatment for a blocked coronary artery in December, turns 91 on Sunday.
He is being treated at King Edward VII Hospital in central London.
A palace spokesman said: “The treatment of his infection continues with antibiotics.
“He is likely to remain in hospital over the next few days. He is in good spirits.”
On Tuesday, the Earl of Wessex, his youngest son, said the duke was “getting better” and had been watching the Jubilee events on television.
Asked how the Queen was coping without her husband, Prince Edward added: “She’s bearing up but missing him, obviously.”
As well as the concert, the duke missed the national service of thanksgiving at St Paul’s Cathedral on Tuesday.
The service was followed by receptions at Mansion House and the Guildhall, a lunch at Westminster Hall and a carriage procession to Buckingham Palace, then by a balcony appearance by the Queen, the Royal Family and a flypast.
In his sermon, the Archbishop of Canterbury said “our prayers and thoughts” are with the duke.