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 that he travelled by car to the hospital, where he was admitted on his doctor’s advice.
The duke had been feeling unwell for a few days, but it was not related to coronavirus, the source added.
It added that he was “in good spirits” in hospital, where is expected to remain for a few days of observation and rest.
The Queen, 94, remains at Windsor.
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”.
The palace is pretty protective of the privacy of members of the Royal Family when it comes to health issues; those hoping for running commentary are invariably disappointed.
But palace spokespeople have gone out of their way to emphasise that this was not an emergency admission and that it was precautionary.
The duke has been in and out of hospital a bit over the past few years.
But overall his later years were marked by his health and energy; he gave up smoking the day he married the Queen; he has never drunk heavily and was for decades a keen sportsman, driving himself constantly to stay fit and strong when others might have been tempted to take it easy.
The last time he was seen in public he seemed in good health, standing unaided – and he walked into hospital on Tuesday evening, having travelled there by car.
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.