The Duke of Edinburgh has been admitted to hospital in central London for planned surgery on his hip, Buckingham Palace has said.
The surgery will take place on Wednesday at King Edward VII's Hospital in Marylebone.
Prince Philip, 96, retired from royal duties last August.
The duke did not attend the Easter service at Windsor Castle on Sunday, with his absence said to have been because of a hip problem.
He also missed two other events at Windsor in recent days - last Thursday's Royal Maundy Service and a parade on 22 March to mark Prince Andrew's appointment as Colonel of the Grenadier Guards.
In a statement, Buckingham Palace said: "His Royal Highness The Duke of Edinburgh was admitted to King Edward VII Hospital in London this afternoon, for planned surgery on his hip which will take place tomorrow.
"Further updates will be issued when appropriate."
The duke announced his retirement in May 2017, after decades of supporting the Queen, as well as attending events for his own charities and organisations.
A month later he was admitted to the private King Edward VII's Hospital as "a precautionary measure" following an infection arising from a pre-existing condition.
His final solo engagement was on 2 August last year, when he reviewed a parade of Royal Marines outside Buckingham Palace in his capacity as Captain General of the corps.
But he has since appeared several times at engagements alongside the Queen.
The surgery comes ahead of a busy time for the royal family, with the Duchess of Cambridge due to give birth to her third child this month, and the wedding of Prince Harry and Meghan Markle in May.
Prince Philip has spent much of his life in good health, but he was treated for a blocked coronary artery in 2011 and a bladder infection in 2012.
In June 2013 he spent 11 nights at the private London Clinic following exploratory abdominal surgery.
In May 2014, he had a "minor procedure" on his right hand, with the treatment carried out inside Buckingham Palace.
By James Gallagher, BBC health and science correspondent
Surgery at any stage of life carries risks.
But patients are not ruled out of surgery based on their age alone.
For patients like Prince Philip, overall fitness is a bigger factor than how old they are.
This will be a decision between patient and surgeon assessing the benefits and the risks of the planned procedure.
In general, frailty increases with age and does increase the risks of major surgery.
We do not know what type of hip operation Prince Philip is having.
Most hip replacement operations in the UK take place on patients between the ages of 60 and 80.
However, such operations do take place even later in life.
Back in 1995, The Queen Mother had a successful hip replacement at the age of 95.