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.
The announcement was made just before 2pm, shortly after the Queen, who caught the 10.42am Great Northern service from King’s Cross station in London, arrived at platform two in King’s Lynn at 12.31pm.
The 93-year-old monarch, carrying a black handbag and wearing a salmon pink coat and patterned headscarf, was beginning the first day of her Christmas retreat after carrying out her duties at the State Opening of Parliament on Thursday.
Other members of the royal family will join the couple for part of the festive period.
Events will include the family’s traditional Christmas Day church service at St Mary Magdalene Church.
As Philip has continued late into his 90s, concerns for his well-being have increased following a number of health scares.
Abdominal surgery, bladder infections and a blocked coronary artery have seen him admitted to hospital on a number of occasions.
Philip suffers from arthritis in his right wrist, and broke a bone in his ankle playing polo. He also developed synovitis, a rheumatic condition of the tendon in the hand, after a polo fall.
In late 2011, the Duke, then aged 90, was rushed to hospital by helicopter from Sandringham after suffering chest pains as the Royal Family were preparing for Christmas.
In the serious health scare, he was treated for a blocked coronary artery at Papworth Hospital in Cambridgeshire and underwent a minimally invasive procedure of coronary stenting, and spent Christmas Day and Boxing Day in hospital.
And in the midst of the Queen’s Diamond Jubilee celebrations in June 2012, the Duke was forced to miss the majority of the festivities after falling ill with a bladder infection.
The Duke has also made several trips to the King Edward VII’s private hospital, in Marylebone, in recent years.
In June 2017, the Duke spent the day at Royal Ascot, where the hot weather meant the dress code in the Royal Enclosure was relaxed for the first time.
That same evening, he was admitted to King Edward VII’s Hospital as a precautionary measure, for treatment for an infection arising from a pre-existing condition.
He missed the State Opening of Parliament, with the Prince of Wales stepping in to accompany the Queen, before being discharged after a two-night stay.
In March 2018, he missed the Maundy Thursday service with the Queen because of a problem with his hip and a few days later, in April, Philip was admitted to hospital ahead of a planned hip replacement.
The Duke left hospital 10 days later.
In January this year, 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.