Prince Philip will remain in hospital for a few more days after he was admitted for a pre-existing condition.
Buckingham Palace said in a statement on Saturday morning that the 98-year-old will be kept under observation.
"The Duke of Edinburgh travelled from Norfolk this morning to King Edward VII's Hospital in London for observation and treatment in relation to a pre-existing condition," it read.
"The admission is a precautionary measure, on the advice of His Royal Highness's doctor. The duke was not taken by ambulance and is expected to be in hospital for a few days."
Local media report that his hospitalisation comes after several bouts of bad health.
The Daily Mail says palace sources have confirmed Philip recently had a "bad fall" and was also hit by a flu-like condition.
"The duke has had a bad turn recently and has been looking much frailer of late," one insider said, according to the Daily Mail.
"He's a remarkable man who has always prided himself on how robust he is, but he is six months off his 99th birthday and age takes its toll. He hasn't been looking terribly well of late."
The Queen has already left for Sandringham, where she reportedly hopes Philip will be able to join her for Christmas.
During Christmas in 2011, the duke underwent surgery for a blocked artery. He was also hospitalised with a bladder infection in 2012, Reuters reports .
Last year, Prince Philip had his hip replaced, requiring a 10-day stay.
Prince Philip is the husband of Queen Elizabeth II. The couple celebrated their 72nd wedding anniversary in November.