The Duke of Edinburgh is responding well to hospital treatment after a recurrence of the bladder infection he had in the summer, Buckingham Palace has said.
Philip was taken to Aberdeen Royal Infirmary while staying with the Queen at her retreat at Balmoral.
In a statement, the palace said: "The Duke of Edinburgh is responding well to treatment. It is unclear when he will be discharged, but he is likely to remain in hospital for a few more 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 on Wednesday, the palace said.
Philip's latest health scare came two and a half months after he spent five nights in hospital with a bladder infection following the Diamond Jubilee Pageant on the River Thames.
He was forced to miss some of the celebrations for his wife's 60 years on the throne after he was admitted to the King Edward VII Hospital in central London the day after braving the elements on a barge for several hours.
Philip also underwent a successful procedure to clear a blocked coronary artery last December and was in hospital for four nights over Christmas.
Philip went to Balmoral after attending a number of Olympic events. He was at the Queen's side during the Opening Ceremony on July 27 and the following day when they toured the Olympic Park. He then watched his granddaughter Zara Phillips' Olympic equestrian debut on July 29 and was at the ExCeL exhibition centre a few days later to watch boxing.
A royal spokesman said: "He has been resident at the castle for a short while but whilst he has been there, he has also undertaken engagements on the Isle of Wight."
It is not known whether or not other members of the royal family, including the Princess Royal, whose birthday it was on Wednesday, were also staying at Balmoral.