Prince Philip was admitted to the hospital on Tuesday afternoon, for what Buckingham Palace has described as planned surgery on his hip.
The Duke of Edinburgh has undergone a successful hip replacement operation and is comfortable and in good spirits, Buckingham Palace has said.
The exact nature of Philip's hip problem was unclear but Buckingham Palace said an update on his health would be given when appropriate.
"Any operation and any anaesthetic carries some risk", she said.
While Prince Philip has been in a fairly healthy condition throughout his life, he had been reportedly treated for a blocked coronary artery in 2011, and a bladder infection in 2012.
The duke stepped down from his public duties last summer, but on occasion does attend events with the Queen.
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He has missed a series of engagements and events over the last month, including the recent Easter Sunday service at St. George's Chapel in Windsor.
He did not accompany Elizabeth to a church service on Maundy Thursday because of the hip problem, a royal source said last week.
"What is most important is someone's fitness, as in their heart and lung fitness, how much exercise they do and how well nourished they are".
Philip has been in generally good health for his age, but he was briefly hospitalised over Christmas in 2011 for angioplasty treatment of a blockage in his coronary arteries.
There's a busy few months coming up for the royal family, as the Duke and Cambridge prepare to welcome their third child, and Prince Harry and Meghan Markle marry next month.
Also invited will be 200 people from various charities associated with the couple, 100 students from two local schools, 610 Windsor Castle community members and 530 Members of The Royal Households and Crown Estate.