The Duke of Edinburgh was admitted to hospital on Tuesday evening “as a precautionary measure” after feeling unwell, Buckingham Palace has said.
Prince Philip, 99, was taken to the King Edward VII Hospital in London, the statement added.
A palace source told the BBC the duke travelled by car to the hospital, where he was admitted on his doctor’s advice.
He had been feeling unwell for a few days, but it was not related to coronavirus, the source added.
They said the duke – who celebrates his 100th birthday on 10 June – was “in good spirits” in hospital, where he is expected to remain for a few days of observation and rest.
The Queen, 94, has remained at Windsor Castle. Earlier, she spoke by telephone with the First Sea Lord, Admiral Tony Radakin, about the work of the Royal Navy at home and abroad.
Last month, the palace announced that Prince Philip and the Queen had received Covid-19 vaccinations from a household doctor at Windsor Castle.
The couple have been spending the coronavirus lockdown in England at Windsor Castle with a small household staff, nicknamed HMS Bubble.
They celebrated Christmas quietly at their Berkshire residence, instead of the traditional royal family gathering at Sandringham.
A No 10 spokesman said Prime Minister Boris Johnson sends his “best wishes” to the duke as he “undergoes a few days of rest in hospital”.
In December 2019, the duke spent four nights at King Edward VII Hospital, where he was admitted as a “precautionary measure” and treatment for a “pre-existing condition”.
He was later discharged on Christmas Eve and driven to the Sandringham estate in Norfolk, where he spent Christmas with the Queen.
The treatment he has received for various health conditions over the years include being treated for a blocked coronary artery in 2011, a bladder infection in 2012 and exploratory surgery on his abdomen in June 2013.
The duke missed events to mark a Battle of Jutland anniversary on “doctor’s advice” in 2016, before retiring from public duties in 2017.
In April 2018, he was admitted to the King Edward VII hospital for a planned hip replacement, and was discharged after nine days.
He waved to the media as he was driven to Windsor Castle to recuperate, before attending the wedding of the Duke and Duchess of Sussex the following month, walking unaided.
He escaped injury after being involved in a car crash while driving near the Queen’s Sandringham estate in January 2019.
His Land Rover Freelander landed on its side after colliding with a Kia as he pulled out of a driveway.
Prince Philip was unhurt, but he visited hospital for a check-up. The Kia’s two passengers required hospital treatment and he later wrote to one of them – Emma Fairweather – who broke her wrist in the accident.
Buckingham Palace later announced that “after careful consideration” he had voluntarily surrendered his driving licence.
Last July, Prince Philip and the Queen attended the private wedding of his granddaughter Princess Beatrice in Windsor. It was believed to be the first time the couple had attended a family gathering since lockdown began.
Later that month, in his first official engagement in over a year, the duke took part in a ceremony at Windsor to hand over his role of colonel-in-chief of The Rifles to the Duchess of Cornwall, who was at Highgrove.
Last November, the Queen and Prince Philip marked their 73rd wedding anniversary by releasing a photograph of themselves opening a card made for them by the Duke and Duchess of Cambridge’s children.