The Duke of Edinburgh has been taken to hospital with a bladder infection, Buckingham Palace said today.
Philip, 90, was taken to the King Edward VII Hospital in London from Windsor Castle earlier today as a precautionary measure.
He will remain in hospital under observation for a few days, a palace spokeswoman said.
The news was released just hours before the Diamond Jubilee concert was due to begin in front of Buckingham palace.
The spokesman said the Queen would still be attending tonight's concert.
The statement about the Duke's health said: "HRH The Duke of Edinburgh was taken to King Edward VII Hospital in London this afternoon, from Windsor Castle, as a precautionary measure after developing a bladder infection, which is being assessed and treated.
"Prince Philip will remain in hospital under observation for a few days.
"He is, understandably, disappointed about missing this evening's Diamond Jubilee Concert and tomorrow's engagements."
An expert has said that it's possible the Duke contracted his bladder infection during the pageant yesterday.
Philip joined his family on the Royal Barge, standing on deck next to the Queen for much of the parade.
But he might not have made enough visits to the toilet while on board the Spirit of Chartwell as it journeyed for more than three hours along the Thames, Dr Malcolm Vandenburg suggested.
If the Duke contracted the infection prior to the boating extravaganza, he might well have been in considerable pain as he waved to crowds, the consultant said.
Dr Vandenburg was unable to pinpoint the precise cause of Philip's condition but said it was most likely to have been brought on by an underlying problem, such as a swollen prostate.
He also cited stress, potentially brought on by the high-profile Jubilee celebrations, saying this could have led the infection to emerge when it did.
The Harley Street doctor, a specialist in general medicine and male health problems, said: "You wonder if he wasn't able to empty his bladder as often as he would have wanted to.
"That would add to the retention of water, meaning he wouldn't have flushed out his system as frequently as he may have wanted.
"If he was having to stand up for four hours, then one would have some concerns that a normal flushing didn't take place."
But he said the infection might have set in at an earlier stage.
"To think he could have had that and stood up all day yesterday makes it all the more remarkable, on the assumption it didn't just start the moment they announced it," Dr Vandenburg said.
"It causes some discomfort in the lower abdomen, it causes some discomfort in the region of the genitals, it causes some discomfort when you pass urine."
A swollen prostate - common among elderly men - was most likely to have brought on the infection, he said.
"If a man gets a bladder infection, it's usually that something else is going wrong as well.
"In a man of around 90, the thing that you would think of is some form of bladder outlet obstruction which would, at the top of the list, be some form of obstruction around the prostate."
This would cause urine to remain in the bladder, causing an infection.
"In 90-year-old men who aren't mentally robust, it can cause them to become very confused and what you would be worried about is the infection travelling upward to reach the kidneys so you want to treat it as soon as possible," Dr Vandenburg said.
But he added: "Given his health and the care he will undoubtedly get, I wouldn't be unduly concerned."
He said the Duke would probably be prescribed antibiotics and a course of drugs to treat the cause of the infection.
Symptoms of a swollen prostate include passing urine too often, discomfort and urine retention, Dr Vandenburg said.