Queen Elizabeth IIs 99-year-old husband Prince Philip has undergone a successful heart procedure at a London hospital, Buckingham Palace said Thursday after he was transferred to a cardiac unit. Royal officials said the Duke of Edinburgh "underwent a successful procedure for a pre-existing heart condition at St. Bartholomews Hospital" on Wednesday. "His Royal Highness will remain in hospital for treatment, rest and recuperation for a number of days," they added. Philip has now spent more than two weeks in hospital, his longest every stay, raising fears for his health because of his advanced age. He will turn 100 on June 10. The prince was first admitted to the private King Edward VII Hospital in central London on February 16. Buckingham Palace said initially that he was expected to stay there for a "few days" after walking in unaided. Later, the palace said that he was being treated for an infection. But on Monday, he was taken to the state-run St. Bartholomews Hospital for testing and observation of a "pre-existing heart condition". Philip retired in 2017 and rarely appears in public. Before his hospitalization, Philip had been isolating at Windsor Castle, west of London, with the queen.