Britain's Prince Philip was hospitalised overnight with a repeat of the bladder infection that made him miss part of his wife Queen Elizabeth II's jubilee celebrations in June, Buckingham Palace said.
The 91-year-old was likely to spend several days in hospital after his third health scare in the space of eight months, having also suffered a blocked coronary artery over Christmas last year.
Buckingham Palace said the Duke of Edinburgh, as he is formally known, was taken by ambulance to the Aberdeen Royal Infirmary in northeast Scotland from Balmoral Castle about 60 kilometres away.
"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," a palace spokesman said.
"His royal highness is likely to remain there for the next few days."
Buckingham Palace earlier said it was a "precautionary measure." It said he and the queen had been staying at Balmoral.
The outspoken prince had visited the Isle of Wight off southern England on Monday and appeared to be in good health as he carried out several public engagements.
He had also watched several events at the London Olympics in the last fortnight, including the Olympic debut of his granddaughter Zara Phillips, who eventually won a silver medal.
Philip was hospitalised with a bladder infection for five nights in June, forcing him to miss the end of the huge diamond jubilee celebrations marking his wife's 60th year on the throne.
The infection came after he and the rest of the royal family spent several hours standing on a royal barge in the pouring rain as part of a jubilee pageant on the River Thames in London.
On the night he was taken ill, hundreds of thousands of revellers chanted his name to wish him well at a star-studded jubilee concert outside Buckingham Palace.
He also spent four days in hospital at Christmastime in December after undergoing a surgical procedure for a blocked coronary artery.
Greek-born Philip is the longest-serving royal consort in British history, and has been a constant fixture by the queen's side throughout her reign.
He is famous for his politically-incorrect jokes, but behind the scenes is said to be a key source of support for the queen.
Born Prince Philippos of Greece and Denmark, Philip fled politically unstable Greece with his family when he was just 18 months old.
A respected officer in Britain's Royal Navy during World War II, he had met Princess Elizabeth in 1939, but the couple did not marry until 1947.
He had to give up his titles as prince of Greece and Denmark and join the Church of England in order to marry the future queen.
He was placed in command of a ship after the wedding, but his career was cut short by the death of king George VI in 1952, which brought Elizabeth to the throne.
He once admitted the curtailment of his career was "disappointing", but said that "being married to the queen, it seemed to me that my first duty was to serve her in the best way I could".
They have four children, including heir to the throne Prince Charles.