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 as a precaution on his doctor’s advice.
The sourced added that the duke is “in good spirits”.
He is expected to remain in hospital for a few days of observation and rest.
The source added that Prince Philip had been feeling unwell for a few days, but that it was not related to coronavirus.
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.
The Queen, 94, remains at Windsor.
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.