The Duke of Edinburgh has been taken to King Edward VII hospital with a bladder infection, Buckingham Palace said.
The Duke, 91 on June 10, was taken to hospital "as a precaution" said the palace.
He had spent last night at Windsor Castle with the Queen.
It comes the day after the Duke and Queen had taken part in the historic Diamond Jubilee pageant along the Thames.
The Duke will now miss tonight's concert at Buckingham.
He will also miss tomorrow's service of thanks giving and the Jubilee fly past.
Prince Philip's absence is likely to overshadow theconcert to a certain degree.
A host of A-list stars from Stevie Wonder to Sir Paul McCartney and Sir Elton John will perform on a stage close to Buckingham Palace in honour of the Queen's 60-year reign.
Many members of the Duke's family will be among the guests, including the Prince of Wales, Duchess of Cornwall, Duke and Duchess of Cambridge and Prince Harry.
But the news that the Queen will still attend and take her seat among the other royals is probably a good sign for monarchy fans.
The Duke appeared to be in good spirits yesterday when he joined the rest of his family on the royal barge for the Diamond Jubilee pageant.
Like the Queen, he stood for most of the 80-minute journey down the Thames and did appear to be in discomfort.
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.
The Buckingham Palace spokesman said Prince Philip will remain in hospital under observation for a few days, a palace spokeswoman said.
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.''
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