Prince Charles and Camilla, the Duchess of Cornwall, are pictured arriving at the London Clinic to visit the Duke of Edinburgh.
They arrived shortly before Prince William and Prince Harry.
The Queen visited Prince Philip in hospital as the Palace described him as "comfortable and in good spirits" following an operation.
The Duke of Edinburgh will celebrate his 92nd birthday, as he did Christmas in 2011, in hospital.
Buckingham Palace say they expect him to remain there for up to a fortnight, but did not say what the exploratory operation was for.