The Duke of Edinburgh has undergone heart surgery in hospital in Cambridge after suffering chest pains.
Buckingham Palace said Prince Philip, who turned 90 in June, was treated for a blocked coronary artery at Papworth hospital in Cambridge on Friday.
The BBC reports a successful "invasive procedure of coronary stenting" was performed.
A stent is a small tube inserted into a blood vessel to keep it open and avoid blockages.
The palace said Prince Philip will remain in hospital under observation for a short period.
The Royal Family had been gathering at Sandringham in Norfolk for Christmas. The hospital is nearby.
Born on the Greek island of Corfu in 1921, Prince Philip served in the Royal Navy in World War II before marrying Elizabeth in 1947.
They celebrated 64 years of marriage in November.
Prince Philip was last in hospital in April 2008, when he spent three days in London with a chest infection.
He pulled out of travelling to Assisi in Italy in October because he had a cold. He had just returned from an 11-day tour of Australia with the Queen.
Next year is the Queen's diamond jubilee.
Papworth is the largest specialist cardiothoracic hospital in Britain 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.