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."
His bladder infection comes after a major health scare over Christmas when he underwent a procedure to clear a blocked coronary artery.
The Duke spent a number of days in hospital after having a stent fitted and appears to have made a good recovery.
He even joked about his health in March while on a Diamond Jubilee visit to a primary school in Harrow, north west London, when he turned down a chance to join a football coaching session, saying his heart was not up to it.