Buckingham Palace has confirmed that Prince Philip is responding well to treatment for his recurring bladder infection.
But despite the 91-year-old's fighting spirit, officials say it is likely the Queen's husband will remain in hospital for a few more days.
The royal has suffered another setback to his health after a keeping up a dizzying schedule that would tire men half his age.
Earlier in the week, the Duke was pictured looking sprightly during the royal family's stay in Scotland.
But on Thursday, he was driven 50 miles in an ambulance to Aberdeen Royal Infirmary from Balmoral after he began feeling unwell during his summer break with the Queen.
A Buckingham Palace spokesperson confirmed: "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."
Philip spent five nights being treated for the infection following the Diamond Jubilee River Pageant.
He was then forced to miss some of the events to celebrate his wife's 60-year reign while he recuperated in King Edward VII Hospital in central London.
Until his recent bout of health problems, the doughty former sailor has been remarkably fit for his age, leading a full and active life and accompanying the Queen on engagements far and wide.
Prince Harry said that minor heart surgery he had in December to clear a blocked artery had given him "a new spurt of life".
His comments came after his grandfather spent four nights in hospital at Christmas after complaining of chest pains while staying in Sandringham.
This new health scare will raise questions about the royal consort's hectic round of duties – he still carries out 350 engagements a year, making him one of the hardest working Windsors.
However, those close to him say the Prince can't think of anything worse than sitting at home quietly.
Just recently the nonagenarian made a number of appearances at the Olympic Games – where he was seen supporting the couple's granddaughter Zara Phillips with Princess Anne (below) – before flying to Scotland where he continued to support his wife.
The royals hosted a tea party for 3,000 guests in Balmoral to mark the end of the Diamond Jubilee celebrations.
Then he headed for Cowes on the Isle of Wight, where he attended a regatta and carried out engagements over several days, before taking a helicopter back to Scotland.
Bladder infections are commonplace among elderly men – and are usually easily treatable with antibiotics – but the Queen is still likely to be worried.
Philip has been her confidant and constant companion for over 60 years of marriage, and the monarch depends on him utterly.
This was evident in the famous speech she gave at an event in 1997, celebrating their golden wedding anniversary.
She said: "He has, quite simply, been my strength and stay all these years, and I, and his whole family, and this and many other countries, owe him a debt greater than he would ever claim or we shall ever know."