The Duke of Edinburgh has been successfully treated for a blocked coronary artery after being rushed to hospital with chest pains, Buckingham Palace said.
Philip was taken from Sandringham to the cardiothoracic unit at Papworth Hospital, Cambridge, last night, where an "invasive procedure of coronary stenting" was performed.
The royal will remain in hospital for observation for a short period.
Buckingham Palace said last night: "Following tests at Papworth Cardiothoracic Hospital in Cambridge this evening the Duke of Edinburgh was found to have a blocked coronary artery which caused his chest pains.
"This was treated successfully by the minimally invasive procedure of coronary stenting. Prince Philip will remain in hospital under observation for a short period."
The Duke is expected to be in hospital for the immediate Christmas period and is likely to be visited by some of the royals gathered in the Queen's private estate in Sandringham.
Dr Simon Davies, consultant intervention cardiologist at the Royal Brompton Hospital in London, said Philip may have been on the verge of a heart attack or actually had one before the stenting procedure was performed.
Dr Davies said: "What they have done is they put a miniature sausage-shaped balloon down the artery, pushed the balloon into the narrowed section and then blown it up.
"That forces the material that is blocking the artery outwards and then gets the blood flowing down the artery again.
"The stent is like a little metal sleeve fitted over the balloon when it is blown up.
"This metallic sleeve is opened up and then when the balloon is deflated and withdrawn the stent stays behind."
This is the most serious health scare suffered by the Duke who is known for being a robust and active 90-year-old.
He has belied his years by carrying on with his many royal engagements and in October joined the Queen for an intensive 11-day tour of Australia that took the royal couple to many of the country's major cities.
The last time he was admitted to hospital for any length of time was in April 2008 when a chest infection laid him low for a number of days and he was eventually admitted for treatment.
Hugo Vickers, a writer and commentator on the royals, told the BBC the Duke would normally think being rushed to hospital was a big fuss.
He said: "He would go to hospital if he needed to, otherwise he would reckon the whole thing was a load of nonsense.
"He's a man who likes to get on with things and he has a wonderful habit of bouncing back from things like this and let's hope he'll do so on this occasion too."
He added that the Duke was "incredibly active" and despite saying ahead of his 90th birthday he wanted to slow down - and indeed stepped down as president or patron of more than a dozen organisations - he has remained busy.
Philip would normally have been welcoming guests to the Queen's private home, which is set in 60 acres of gardens, offering the perfect sanctuary for the family's break.
Christmas Eve is an important day for the royals as they follow the German tradition of opening their presents the day before Christmas - something Queen Victoria and Prince Albert did.
The festive weekend will also see the Duchess of Cambridge spending her first Christmas as a member of the Royal Family.
A large media presence is expected at Sandringham on Christmas Day when the Queen and her family make the short journey to St Mary Magdalene Church on the royal estate for a traditional service.
Back at the house after the church service, lunch is served at 1pm and the family enjoy a giant turkey, reared at Sandringham.