The Queen's husband Prince Philip was taken to hospital this morning, just days before Christmas.
The Duke of Edinburgh has travelled from Norfolk to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition, Buckingham Palace said.
In a statement, the palace said: "The Duke of Edinburgh travelled from Norfolk this morning to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition.
"The admission is a precautionary measure, on the advice of His Royal Highness' Doctor."
The Duke, 98, retired from royal duties two years ago after decades of supporting the Queen and attending events on behalf of the Royal Family.
The news, as reported by The Mirror , comes on the same day as the Queen travelled to Sandringham to begin her Christmas celebrations.
Prince Philip currently lives on the royal estate in Sandringham.
The Duke of Edinburgh and the Queen will be joined by other members of the Royal Family for the festive celebrations.
They will celebrate with Prince Charles, Camilla, the Duke and Duchess of Cambridge, George, Charlotte and Louis on the big day.
The Duke and Duchess of Sussex, along with baby Archie, will be spending time with Meghan's mum Doria.