The Duke of Edinburgh is being treated in hospital after a recurrence of the bladder infection which forced him to miss some of the Queen's Diamond Jubilee celebrations.
Philip was taken to Aberdeen Royal Infirmary while staying with the Queen at her retreat at Balmoral.
In a statement, Buckingham Palace said: "The Duke of Edinburgh has had a recurrence of the bladder infection that he had in the summer and has been admitted to hospital for investigation and treatment.
"His Royal Highness is likely to remain there for the next few days."
The 91-year-old royal made the journey of almost 50 miles from the castle in the Cairngorms to the city hospital by road in an ambulance.
Philip spent five nights in the King Edward VII Hospital in June, suffering from a bladder infection.
Last December, he underwent a successful procedure to clear a blocked coronary artery and was in hospital for four nights over Christmas.