The Duke of Edinburgh has been admitted to hospital for treatment relating to a pre-existing condition.
Prince Philip, who is 98, left Sandringham to attend the King Edward VII Hospital in London on Friday morning.
The announcement came as the Queen arrived at the Norfolk estate after travelling from the capital on Friday.
"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," a Buckingham Palace statement said.
"The admission is a precautionary measure, on the advice of His Royal Highness' Doctor."
The Duke is expected to be in hospital for a few days.
He was not taken by ambulance and it was a planned admission.
The announcement was made just before 14:00 UK time.
In January this year, Prince Philip was involved in a car crash when the Land Rover Freelander he was driving flipped over after colliding with another car near the Sandringham estate.
Prince Philip's military appointments
The Duke of Edinburgh served in the Royal Navy until July 1951, when he ended his naval career.
He is still very close to the Armed Forces and was appointed Admiral of the Sea Cadet Corps, Colonel-in-Chief of the Army Cadet Force and Air Commodore-in-Chief of the Air Training Corps, in 1952.
The following year, in 1953, Prince Philip was promoted to Admiral of the Fleet and appointed Field Marshal and Marshal of the Royal Air Force.
On his 90th birthday in 2011, he was appointed Lord High Admiral by the Queen.
The Duke has also been Colonel-in-Chief, or Colonel, of a number of British and overseas regiments.
Cover image: Prince Philip at the Field of Remembrance service in 2013 (Picture: Crown Copyright).