The Queen's husband Prince Philip, 98, 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 confirmed the news on Friday afternoon.
In a statement, the palace said it was "on the advice of His Royal Highness' Doctor".
The Duke of Edinburgh has been admitted to hospital as a "precautionary measure”.
He was not taken by ambulance and it was a planned admission.
The Duke is expected to be in hospital for a few days.
Prince Philip health update: The Duke of Edinburgh has been taken to hospital
Prince Philip health update: The Duke was admitted to hospital on Friday
The news came shortly after Queen Elizabeth II arrived at King's Lynn railway station in Norfolk.
She was travelling from London to start her traditional Christmas break, which will be spent on the Royal estate at Sandringham.
The Queen’s husband retired from royal duties more than two years ago, in August 2017.
Since retiring from official solo duties, he has appeared in public alongside the Queen and other members of the Royal Family at events and church services.
Prince Philip health update: The 98-year-old Duke retired in 2017
What is wrong with Prince Philip?
Exactly what is wrong with Prince Philip has not been made clear by the palace.
The full 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.”
However, in June 2012, he was treated at the same hospital for a bladder infection.
Back then, the palace said in a statement: "HRH The Duke of Edinburgh was taken to King Edward VII Hospital in London this afternoon, from Windsor Castle, as a precautionary measure after developing a bladder infection, which is being assessed and treated.
"Prince Philip will remain in hospital under observation for a few days.
"He is, understandably, disappointed about missing this evening's diamond jubilee concert and tomorrow's engagements."
Prince Philip health update: The Queen and Prince Philip have been married since 1947
In April 2018, the Duke of Edinburgh also had a hip operation at the same hospital.
Buckingham Palace said he would continue his recovery at Windsor: “His royal highness would like to convey his appreciation for the messages of good wishes he has received.”
For about a month before the operation, Philip had complained about trouble with the joint.
Medical experts have said a few weeks after a hip replacement patients are usually back to their normal lifestyle, free from pain.