The Duke of Edinburgh is responding well to hospital treatment after a recurrence of the bladder infection he had in the summer, Buckingham Palace said today.
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 yesterday, 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.
Before the heart scare he had been fit and apparently healthy, and led the active life of a man of younger years.
In March this year his grandson Prince Harry said the operation had given him "a new spurt of life".
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."
The Duke appeared to be in good spirits as he took part in events as part of Cowes Week on the Isle of Wight, in his role as Admiral and member of the Royal Yacht Squadron.
He viewed the racing in the Solent on Tuesday before attending a luncheon on board a yacht, the My Farragon, accompanied by the Lord Lieutenant of the Isle of Wight, Major General Martin White.
On Monday he had been at a luncheon held at Newport's Chamber of Commerce, then visited manufacturers A.J.Wells and Sons in the town before having dinner at the Pavilion in Cowes.
While in Scotland earlier this month he supported the Queen at engagements including hosting a tea party at Balmoral.
Philip joined the Queen and the Earl and Countess of Wessex at the event on the estate which was attended by around 3,000 guests.
The royals, including the Duke, reportedly looked relaxed during the event, which rounded off the Queen's Diamond Jubilee celebrations.
It was said to have been only the second time that the Queen's Deeside home, where the Royal family prize their privacy, was used for such an event.
It is not known whether or not other members of the royal family, including the Princess Royal, whose birthday it was yesterday, were also staying at Balmoral.