The Duke of Edinburgh is in hospital tonight ahead of an exploratory operation on his abdomen. Buckingham Palace says Philip is expected to remain at the London Clinic for up to two weeks but gave no further details about the procedure.
The planned admission came after this afternoon's garden party at the palace. The operation will take place on Friday.
He is in very good health. He felt unwell on Monday and missed an engagement but that was down to the fact he had temporarily lost his voice.
Lord-Lieutenant of Nottinghamshire Sir John Peace went to today's garden party where he said Philip had been "telling jokes" - but he would not divulge what they were about.
Prime Minister David Cameron sent Philip his good wishes, writing on Twitter:
My best wishes to the Duke of Edinburgh who is in hospital tonight. I hope he has a swift recovery.
The operation will be done under general anaesthetic in line with any operation of this kind. Buckingham Palace could not confirm if the Queen or any other members of the royal family are due to visit Philip in the coming days.
Other than Monday's cancellation, he had carried out this week's engagements as planned and today showed no sign of discomfort as he laughed and joked with guests.
Philip was at the Queen's side at Westminster Abbey on Tuesday for the 60th anniversary of her coronation.
Yesterday, he toured a Victorian steamship, the SS Robin, in London's East End.
Earlier today, the Queen invested Philip with New Zealand's highest honour, making him an additional member of the Order of New Zealand to mark the Diamond Jubilee.
For most of his life, Prince Philip has enjoyed excellent health. His rare trips to hospital were usually for sporting injuries. It is only in recent years that his age has started to catch up with him, as our Medical Editor Lawrence McGinty explains.