Queen Elizabeth II paid a quick visit on Saturday to her husband, who is recovering from a heart operation at a hospital in Cambridge.
Prince Philip, 90, was taken to the hospital from the queen's Sandringham estate in Norfolk late Friday after experiencing chest pains. He had a coronary stent put in after tests found a blocked artery was to blame, though the palace has refused to say if he suffered a heart attack.
Elizabeth was flown in by helicopter and arrived at Papworth Hospital, some 70 miles (113 kilometers) from London, in a Range Rover. She was accompanied by two of her children, Princess Anne and Prince Edward. After a short visit, the royals were seen boarding the helicopter back to Sandringham.
Buckingham Palace said Philip "had a good night."
Doctors said Philip could have suffered a heart attack, but without more information it was impossible to know for sure.
Coronary stenting is standard procedure both to fend off a heart attack or save a patient already in the midst of one, said Dr. Allan Schwartz, chief of cardiology at New York-Presbyterian, Columbia University Medical Center.
Philip has been known to enjoy good health throughout his life and rarely misses royal engagements. Upon his 90th birthday in June, he announced plans to cut back his official duties.
Britain's Prime Minister David Cameron offered his support to Philip.
"The Prime Minister has been kept informed of the situation and wishes the Duke of Edinburgh a very speedy recovery," Cameron's office said.
The palace said it does not know when Philip will be released, only that he is having a "short stay" in the hospital.
Philip had been at Sandringham, the queen's sprawling rural estate in Norfolk, since Monday for the royal family's Christmas festivities.
The palace said Queen Elizabeth II and the royal family will attend church as usual on Sunday.
Another key part of the royal family's Christmas celebrations is the queen's annual message to the nation, which this year will focus on family and community.
The queen has made a prerecorded Christmas broadcast on radio since 1952 and on television since 1957. She writes the speeches herself, and the broadcasts mark the rare occasion on which the queen voices her own opinion without government consultation.
Cassandra Vinograd can be reached at http://twitter.com/CassVinograd