Prince Philip, Queen Elizabeth's husband, had an emergency heart procedure on Friday evening to relieve a blocked coronary artery, royal officials said.
He was taken to hospital from Sandringham, the Queen's sprawling estate in rural Norfolk, after experiencing chest pains, a spokeswoman for Buckingham Palace said.
After initial tests at Papworth Hospital in Cambridge, the prince underwent a "very minimally invasive procedure" to implant a stent in the blocked artery and will remain in hospital for "a short time" for observation, the spokeswoman said.
She declined to comment further and spoke on customary condition of anonymity. A hospital spokeswoman referred all calls back to the palace.
Philip is known for his good health and rarely misses royal engagements. Upon his 90th birthday in June, he announced plans to cut back his official duties.
The colourful and often outspoken husband of Queen Elizabeth has been a familiar figure at his wife's side for decades. He has championed numerous charities over the years, but is advising the ones he heads to start planning an orderly transition as he plots the end of his working life.
Philip has been at Sandringham since Monday for the royal family's Christmas festivities, Buckingham Palace said.
The royals will celebrate a traditional Christmas this weekend, followed by a year's worth of festivities to mark the Queen's 60th year on the throne.
The Queen and Prince Philip are planning to mark the event with a series of tours throughout England. It is to culminate with a celebration in London in early June that will include an unprecedented pageant on the River Thames with up to 1,000 boats taking part.
Most of the senior royals will be dispatched across the globe to help the Queen celebrate her Diamond Jubilee.
Prince Charles and wife Camilla, the Duchess of Cornwall, will visit New Brunswick, Ontario and Saskatchewan in May, as part of a world tour that will also take in Australia, New Zealand and Papua New Guinea.
Charles's eldest son, Prince William, will visit Malaysia, Singapore, the Solomon Islands and Tuvalu on behalf of the monarch with wife Kate Middleton, now formally known as the Duchess of Cambridge.