"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".
It was not revealed what the pre-existing condition was, but the royal was admitted to the same hospital in April a year ago for an operation on his hip.
The Queen appeared in good spirits as she attended a carol service on Sunday while the Duke of Edinburgh prepared to spend a third night in hospital.
While the palace has downplayed the Duke's admission as "pre-planned", a royal expert claimed the Queen's reaction suggests that it's business as usual for the monarch herself.
The duke's admission on Friday to the private hospital, which has treated royals for decades, was a "precautionary measure", said Buckingham Palace.
The duke was not taken by ambulance and was expected to be in hospital for a few days.
The Queen arrived at Sandringham after leaving London on Friday.
She caught the 10:42 GMT Great Northern service from London's King's Cross and was later pictured stepping off the train at King's Lynn railway station.
It is unclear if Prince Philip will be released from the hospital in time to join the rest of the royal family for Christmas at the queen's country estate in Norfolk.
If he's discharged from the hospital in time, Philip will be part of the queen, Prince Charles, Duchess Camilla, Prince William, Duchess Kate and their children - Prince Louis, 6, Princess Charlotte, 4, and Prince Louis, nearly 20 months - to have a good time Christmas Eve and Christmas in Sandringham.
Guests for Christmas include Prince Charles and Camilla, Prince William and Kate Middleton and the Duke and Duchess of Cambridge's children, Prince George, Princess Charlotte and Prince Louis.
Since 1952, The Duke of Edinburgh carried out thousands of appearances alongside Her Majesty.
The duke has not been seen in public since attending Lady Gabriella Windsor's royal wedding in May this year.
His decision to retire from public duties during 2017 was not health-related, Buckingham Palace said at the time of the announcement.