Prince Philip is being treated at King Edward VII hospital in central London for an infection arising from a pre-existing condition.
A Buckingham Palace spokeswoman said the 96-year-old would stay in the private hospital overnight, but continued to be in good spirits.
The BBC understands his hospital stay is "not a cause for serious concern".
The duke was with the Queen at Trooping the Colour on Saturday and the opening day of Royal Ascot on Tuesday.
