The Duke of Edinburgh has spent the night in hospital after being admitted for treatment relating to a "pre-existing condition".
The 98-year-old travelled from Norfolk to the King Edward VII Hospital in London, said the Palace.
Philip is expected to spend the run-up to Christmas being cared for by medical staff.
In a statement, Buckingham Palace 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."
- ITV News Correspondent Sejal Karia with the latest from outside the King Edward VII Hospital
He was not taken by ambulance and it was a planned admission.
The Queen had earlier on Friday travelled to Norfolk to begin her Christmas break at the Sandringham estate.
She caught the 10.42am Great Northern service from King’s Cross station in London and arrived at platform two in King’s Lynn on time, at 12.31pm.
The 93-year-old monarch, carrying a black handbag and wearing a salmon pink coat and patterned headscarf, stepped from the front carriage of the train on to the platform.
She briefly mingled with passengers before she was escorted through a side exit known as the Royal Gate to a waiting Range Rover.
Sandringham, the Queen’s private estate in Norfolk, is a 30-minute drive from King’s Lynn.
The Queen’s arrival at Sandringham on a Friday is a change from recent years when she has tended to travel on a Thursday.
Passenger Monika Saganowska, 25, who boarded the train at Watlington, the stop before King’s Lynn, said she was unaware the Queen was on board until she arrived at King’s Lynn and saw police on the platform.
She said it was “quite exciting” to discover that the Queen had been on the service, and she was surprised to learn that the monarch had taken public transport.
“I think it’s great actually,” said Ms Saganowska.
The Queen and the Duke of Edinburgh, 98, who spends much of his time at Sandringham since he retired from public duties in 2017, will be joined by other members of the royal family for part of the festive period.
Events will the family’s traditional Christmas Day church service at St Mary Magdalene Church.
Prince George and Princess Charlotte could attend the service for the first time this year, according to reports.
However, the Duke and Duchess of Sussex will be absent, instead opting to spend their first Christmas as parents with the Duchess’s mother Doria Ragland.
Prince Philip's health has been the subject of much speculation over recent years, since he retired from public life.
He underwent hip surgery in the same hospital in April 2018.
After spending 10 days there, Philip is seen waving to the waiting media as he is discharged.
He was also treated there in June 2017, again as a precaution for a pre-existing condition.