Prince Philip, the 90-year-old husband of Queen Elizabeth II, was hospitalised overnight with a bladder infection and will miss the rest of her diamond jubilee celebrations, Buckingham Palace said.
The new health scare comes less than six months after the outspoken British royal patriarch suffered chest pains during the Christmas break and had to undergo a medical procedure to treat a blocked coronary artery.
"The Duke of Edinburgh was taken to King Edward VII Hospital in London this afternoon, from Windsor Castle, as a precautionary measure after developing a bladder infection, which is being accessed and treated," the palace said in a statement.
"Prince Philip will remain in hospital under observation for a few days. He is, understandably, disappointed about missing this evening's Diamond Jubilee Concert and tomorrow's engagements."
The queen, 86, is due to light the centrepiece in a chain of 4,200 beacons spanning the Commonwealth following a star-studded concert in front of Buckingham Palace on Monday featuring Elton John, Tom Jones, Paul McCartney, Shirley Bassey and Stevie Wonder.
She will now also be without her husband during a ceremonial carriage procession through the streets of London on Tuesday that will round off the jubilee festivities.
Prince Philip, the longest serving royal consort in British history, will turn 91 on Sunday but will likely mark the occasion with his characteristic lack of fuss.
He appeared in good health on Sunday as he travelled on the royal barge in chilly conditions during a 1,000-boat river pageant for the jubilee. The former Royal Navy commander was seen saluting to passing boats, smiling and waving during the pageant.
He was also in animated form at the Epsom Derby races on Saturday, waving his top hat to the crowds as he arrived and pointing things out from the royal balcony to his grand-daughters, Princesses Beatrice and Eugenie.
Revellers attending last night's concert were dismayed to hear the news of his illness.
"He's almost as important to the monarchy as the queen is," said Nigel Hainsworth, who was camping out on The Mall, the ceremonial route to Buckingham Palace. "He's been by her side for 60 years."
"He's been her rock, there's no doubt about that," added Hainsworth's father John.
"I'm sorry he's going to miss it," said head teacher Laura Fishleigh. "After 60 years doing what they've been doing, you ought to get a bit of a celebration. It's like missing your birthday party."
On December 23, Philip was admitted to hospital after complaining of chest pains, and was found to have a blocked coronary artery. He underwent surgery to have a stent fitted before being released on December 27.
It was the most serious health scare to date for Philip. He has been married to the queen for 64 years and they have four children together, including heir to the throne Prince Charles.
Prince Philip announced on his 90th birthday last year that he was cutting back on his public engagements after years of touring the globe several footsteps behind his wife.
He travelled with the queen to Australia in October 2011 although he pulled out of a visit to Italy just afterwards because he was nursing a cold.
In her diamond jubilee address to both houses of parliament in March, the queen paid homage to her husband in a rare public tribute that was nonetheless humorous and affectionate.
"During these years as your queen, the support of my family has, across the generations, been beyond measure," she told lawmakers.
"Prince Philip is, I believe, well-known for declining compliments of any kind. But throughout he has been a constant strength and guide."
The Greek-born prince has become a national institution - almost as much for his plain-speaking as for his support for the queen.
On a visit to China in 1986, he warned a group of British students: "If you stay here much longer, you'll all be slitty-eyed." He told a British student who had trekked in Papua New Guinea in 1998: "You managed not to get eaten, then?".