Prince Philip has been admitted to hospital, Buckingham Palace has said.
The palace said Philip, 99, went into hospital on the advice of his doctor on Tuesday evening.
The palace explained in a note: "The Duke’s admission is a precautionary measure, on the advice of His Royal Highness’s Doctor, after feeling unwell.
"The Duke is expected to remain in hospital for a few days of observation and rest."
Philip is in the King Edward VII Hospital in London and is said to be in "good spirits".
A No 10 spokesman said: “The Prime Minister sends his best wishes to the Duke of Edinburgh as he undergoes a few days of rest in hospital.”
Prince Philip has been living in Windsor Castle for several months, following a brief stay in his usual home at Wood Farm in Sandringham last summer.
He retired from royal duties in 2017 at the age of 95, and moved to Norfolk where he keeps up with his patronages and paints.
However he was flown down to Windsor in March 2020, before the UK went into the first coronavirus lockdown, to be close to his wife, the Queen.
They have remained there for most of the time ever since, bar a short break to Balmoral, and Sandringham in the summer. Philip came back with her.
He has made a few statements throughout the year in support of charities he has represented over the course of his royal life.
And he has appeared in a number of photographs, most recently as he and his wife looked over their anniversary cards, marking 73 years of marriage.
Philip also carried out a rare engagement in July 2020, as he appeared outside Windsor Castle for the ceremony to transfer his role of Colonel-in-Chief of The Rifles to his daughter-in-law, Camilla, the Duchess of Cornwall.
The Queen and the Duke have both received the first of their COVID-19 vaccines earlier this year.
The duke was last in hospital just before Christmas in 2019, when he was taken in on his doctor's advice as a precaution.
He was there for four days before leaving in time for Christmas at Sandringham.
He had a hip operation in September 2018 at the same hospital and was there for 10 days. It was a planned operation.
In June 2017, he spent three days in the hospital with an infection.
Plans are likely to be underway for the duke's 100th birthday, which is in June. He is unlikely to want anything over the top, but some marker of the occasion will be made.
However it's been reported the celebration might go ahead without his grandson Prince Harry, who is in California, as Meghan Markle is expecting their second child.
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