The Duke of Edinburgh might still be in hospital on Sunday, his 91st birthday.
Prince Philip was admitted to King Edward VII Hospital on Monday after watching the river pageant, part of the Queen's Diamond Jubilee on Sunday.
On Wednesday he was visited by the Queen and Prince Andrew.
Buckingham Palace said earlier this week that he was being treated with antibiotics and was in good spirits.
Prince Philip underwent a successful procedure to clear a blocked coronary artery last December when he was in hospital for four nights over Christmas.
He had been fit and apparently healthy before this, and led the active life of a man of younger years.
His next scheduled engagement is on Tuesday, when the Queen hosts a garden party at Sandringham to mark her Diamond Jubilee.