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”.