The Duke of Edinburgh has been taken to hospital after suffering chest pains, Buckingham Palace has said.
Prince Philip was treated for a blocked coronary artery and a successful “invasive procedure of coronary stenting” was performed.
The duke, aged 90, had been taken from Sandringham to the cardiothoracic unit at Papworth Hospital in Cambridgeshire.
The Royal Family had been gathering at Sandringham, a traditional royal retreat in Norfolk, for Christmas.
The palace said that following tests the duke was found to have a blocked coronary artery which had caused his chest pains.
This was treated successfully by the minimally invasive procedure of coronary stenting.
Prince Philip will remain in hospital under observation for a short period, it added.
The procedure involves pushing a balloon into the artery and blowing it up to remove the blockage.
The stent is a metal sleeve fitted over the balloon, which remains inside the body when the balloon is removed.
The duke would have remained conscious during the procedure.
The BBC’s Peter Hunt said the Greek-born prince was in strikingly good health and there had been no suggestion of ill health.
The prince attended a lunch for his staff a week ago and was reportedly “on very good form”, he added.
No other member of the Royal Family travelled with him to the hospital. According to Sky News, he was flown there in a helicopter.
Senior members of the Royal Family, including the Prince of Wales and the Duke and Duchess of Cambridge, will arrive at Sandringham on Saturday for Christmas.
Our correspondent said it was far from clear whether Prince Philip would join them or be confined to his hospital bed.
Papworth is the UK’s largest specialist cardiothoracic hospital and the country’s main heart and lung transplant centre.
It treats more than 22,800 inpatient and day cases and 53,400 outpatients each year.
Prince Philip, who arrived at Sandringham on Monday, is the longest-serving royal consort and celebrated 64 years of marriage in November. (BBC)
He was last in hospital in April 2008, when he spent three days in London with a chest infection.
In October this year he pulled out of travelling to Assisi in Italy because he had a cold.
He had just returned from an 11-day tour of Australia with the Queen.
Royal commentator Margaret Holder said: “He is 90, he has been in remarkable health. For somebody who has just done an 11-day tour of Australia at 90, that’s amazing in itself as it is.”
She said the long flights may have “taken something out of him”.