The Duke of Edinburgh's condition has "improved considerably" since he was admitted to hospital on Monday with a bladder infection, Buckingham Palace has said.
However he is expected to be kept under observation for at least the next few days.
A spokesman added: "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."
Philip, 90, is suffering from a bladder infection and is being treated at King Edward VII Hospital in central London.
The illness forced the Duke to miss several key events of the Queen's Diamond Jubilee celebrations, including a service of thanksgiving at St Paul's Cathedral and the concert outside Buckingham Palace.
The Duke of Wessex, who visited him on Wednesday, said his father was "feeling a lot better" and had been watching events unfold on television.
When asked how The Queen was coping without her husband, he said: "She's bearing up but missing him, obviously."