The Duke of Edinburgh has been taken to hospital after feeling "unwell", according to Buckingham Palace.
The 99-year-old was taken to the King Edward VII Hospital in London on Tuesday evening.
The Palace said in a statement that Prince Philip's admission was "a precautionary measure, on the advice of His Royal Highness's Doctor, after feeling unwell".
"The duke is expected to remain in hospital for a few days of observation and rest," the statement added.
It is understood that the royal travelled by car and it was not an emergency or a case of Covid-19.
The Queen, 94, remains at Windsor, where she has been since Christmas, with the royals skipping their usual winter trip to their Norfolk home in Sandringham because of the pandemic.
The couple were among the first in the UK to receive their first dose of the Oxford-AstraZeneca vaccine at Windsor Castle in January.
The Duke of Edinburgh is due to celebrate his 100th birthday on June 10.
Philip was flown by helicopter from Norfolk to the same hospital just over a year ago in December 2019, for treatment for a "pre-existing condition".
Buckingham Palace would not go into details about his ailment or the nature of his treatment at the time.
The hospital dash followed a month of ill health for the Duke, including a nasty fall.
At the start of 2019, the Duke was involved in a car crash when the Land Rover Freelander he was driving flipped over after a collision with another vehicle near the Sandringham estate.
He was given medical treatment and voluntarily surrendered his driving licence, after photos emerged of him driving a vehicle at Sandringham without a seatbelt.
Prince Philip decided to retire from public duties in 2017, but the Palace said at the time that the move was not health-related.
His health scares have included heart problems, arthritis, abdominal surgery and bladder infections.
In April 2018 he had a hip replacement operation but he had recovered well for Prince Harry and Meghan's wedding a month later.
Prince Philip and the Queen celebrated their 73rd anniversary in November.