The Duke of Edinburgh could have been "on the verge of" suffering a heart attack, an expert said.
He was taken from Sandringham to the cardiothoracic unit at Papworth Hospital, Cambridge, where an "invasive procedure of coronary stenting" was performed.
Dr Simon Davies, consultant interventional cardiologist at the Royal Brompton Hospital in London, said: "It meant that the pain was from the heart. It means that one or more of the coronary arteries was badly narrowed or perhaps blocked.
"That meant that the blood was not passing through that artery so the muscle was starving of oxygen and in danger of dying, in other words a heart attack, or was on the verge of one."
He added that the blockage was likely to have been made up of cholesterol and a blood clot and Philip may be given blood-thinning tablets.
Dr Davies said it was "impossible to say" how long Philip could be kept in hospital.
But he added: "The fact is he must have looked after himself to be looking so well."
One medical expert said that many patients are out of hospital just a day after having the treatment.
Asked if there was a chance the Duke could be discharged in time for Christmas, Dr David Lloyd told Sky News: "It all depends on complications, but yes, if it's a straightforward thing you can be out the next day."
Dr Lloyd said of the procedure: "It looks simple, but in fact it's incredibly clever. I mean the fact that it is able to expand the artery and then stay expanded, and then the tissue grows over the top of it once it's sitting in there, is a really magnificent thing.
"It's the culmination of years and years and years of research to produce these remarkable stems."
He said only a local anaesthetic was needed for it to be carried out.
The GP added: "You're on the table, you're awake, and a tube is inserted in one of your arteries either in your arm, or in your leg, and then after a couple of hours lying to stop the bleeding after the artery's been pierced, you're up and about again."Suggest a correction