The Duke of Edinburgh has received treatment for a blocked coronary artery, Buckingham Palace has said.
A "minimally invasive procedure of coronary stenting" was successfully performed after he was taken to hospital suffering chest pains.
The duke, aged 90, is said to have had a "good night" at Papworth Hospital in Cambridgeshire.
The Royal Family had been gathering at Sandringham, a traditional royal retreat in Norfolk, for Christmas.
Members of the family are expected visit him later in hospital.
Prince Philip will remain in hospital under observation for a short period, it added.
The coronary stenting 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.
He was flown to the hospital by helicopter. No other member of the Royal Family travelled with him.
Papworth is the UK's largest specialist cardiothoracic hospital and the country's main heart and lung transplant centre, treating more than 22,800 inpatient and day cases and 53,400 outpatients each year.
Senior members of the Royal Family, including the Prince of Wales and the Duke and Duchess of Cambridge, were due to arrive at Sandringham on Saturday for Christmas.
Prince Philip, who arrived at Sandringham on Monday, is the longest-serving royal consort and celebrated 64 years of marriage in November.
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.
A Downing Street spokesman said: "The prime minister has been kept informed of the situation and wishes the Duke of Edinburgh a very speedy recovery."
Meanwhile, the Queen's Christmas Day broadcast to the nation will highlight the importance of the family unit. The speech, taped on 9 December, reflects on a year in which her grandson Prince William got married and her husband turned 90.