The Duke of Edinburgh has been admitted to hospital for treatment relating to a pre-existing condition, Buckingham Palace said.
Philip, 98, left Sandringham, where he spends much of his time since retiring, 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.
A Buckingham Palace statement 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 is expected to be in hospital for a few days.
He was not taken by ambulance and it was a planned admission.