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 central 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 is due to celebrate 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.
Prince Charles and the Duchess of Cornwall also carried out official duties on Wednesday, visiting the Queen Elizabeth Hospital in Birmingham to speak to patients and NHS staff.
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 recent 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 family gathering at the Queen’s Sandringham estate in Norfolk.
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”.