A palace source confirmed it was not Covid-related and said the stay in hospital was "purely precautionary". The source stressed that his arrival was "not an emergency admission", and that he had been "feeling unwell" for a short period prior to his admission.
Both Prince Philip and Queen Elizabeth, 94, received their first COVID-19 vaccinations in January.
The palace says the husband of Queen Elizabeth II was admitted to the private King Edward VII Hospital on Tuesday evening.
And it's because Prince Philip is just a few months away from his 100th birthday that this hospital admission can not be taken too lightly, despite what the Palace aides are advising.
"The Duke's admission is a precautionary measure, on the advice of His Royal Highness's Doctor, after feeling unwell".
The couple have been spending the coronavirus lockdown in England at Windsor Castle with a small household staff, nicknamed HMS Bubble.
Ash Barty's classy response to opponent's timeout controversy
Muchova came out of the break and broke Barty the next game, who lost all four points on her racket with unforced errors. Muchova's only other victory over a Top 5 player was against No. 3 Karolina Pliskova at Wimbledon in 2019.
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Philip spent four nights at King Edward Hospital in December 2019, where he was treated for what was described as a "pre-existing condition" and though he was discharged on Christmas Eve that year, he has not been seen in public since.
Her Majesty will celebrate her 95th birthday in April, while Philip is set to mark his 100th birthday on 10 June. Philip also skipped Easter Sunday services that year ahead of the announcement that he underwent hip replacement surgery.
Philip made a rare appearance in public last July at a brief ceremony at Windsor Castle.
Earlier this week, he celebrated the news that grandson Prince Harry and Meghan Markle announced that they are expecting their second child.
The queen remains at Windsor, the source said.
The duke was then driven to Sandringham and spent Christmas with the Queen.