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 the King Edward VII Hospital in Marylebone.
Prince Philip, 96, retired from royal duties last August.
The duke was not at last Thursday's Royal Maundy Service at Leicester Cathedral, with his absence said to be because of a hip problem.
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 King Edward VII hospital as "a precautionary measure" following an infection arising from a pre-existing condition.