The Duke of Edinburgh has spent the night in hospital after being admitted for treatment relating to a "pre-existing condition".
Philip is expected to spend the run-up to Christmas being cared for by medical staff after being admitted to the King Edward VII's Hospital in central London.
Philip's planned admission on Friday to the private central London hospital, which has been treating members of the royal family for decades, was described as a "precautionary measure" by Buckingham Palace.
According to reports, his admission follows a spell of ill-health – the Sun quoted a royal source saying the duke had a fall recently, while the Mail reported he had been battling a flu-like condition.
Uniformed police officers stood guard outside the hospital entrance on Saturday morning alongside a doorman wearing a top hat while camera crews were set up across the road.
The Queen did not change her schedule and left Buckingham Palace for Norfolk by train, to begin her traditional festive break at her private Sandringham estate, where the duke spends much of his time since retiring from public duties.
The duke, who turned 98 in June, has generally enjoyed good health, appearing to recover well from a planned hip replacement operation in April 2018.
Philip escaped with minor injuries after a dramatic car crash near Sandringham in January, but in the past decade has been admitted to hospital for abdominal surgery, bladder infections and a blocked coronary artery.
A Buckingham Palace statement on Friday 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's doctor."
The duke was not taken by ambulance and is expected to be in hospital for a few days.
Philip is known for his "no fuss" approach and is unlikely to receive many visitors while he is being treated at the hospital.
Buckingham Palace would not go into details about Philip's pre-existing condition, or the nature of his treatment, but there is likely to be concern for the duke among well-wishers given his age.
His decision to retire from public duties during 2017 was not health-related, Buckingham Palace said at the time of the announcement.
The duke is an advocate of healthy eating combined with exercise. He once said he more or less followed the Atkins diet and he drinks only moderately.
He has given up riding carriages competitively, but earlier this year was photographed in the grounds of Windsor Castle holding the reins as he guided a horse and carriage through the estate.