The Duke of Edinburgh has been admitted to hospital for an exploratory operation on his abdomen, Buckingham Palace said tonight.
Philip is expected to remain at the London Clinic for up to two weeks.
The Palace said in a statement: "His Royal Highness The Duke of Edinburgh has been admitted to the London Clinic for an exploratory operation following abdominal investigations. He is expected to stay in hospital for up to two weeks.
"Further updates will be issued when appropriate."
Philip's admission to hospital was shortly after he attended a Buckingham Palace garden party on Thursday afternoon.
The Duke, who turns 92 on June 10, had missed an engagement at the palace on Monday night because he was feeling "under the weather". However a Buckingham Palace spokeswoman said his admission to hospital was not related to this and merely because he had lost his voice.
Lord-Lieutenant of Nottinghamshire Sir John Peace attended Thursday's garden party where he said Philip had been "telling jokes" - but he would not divulge what they were about.
Speaking of meeting the Queen at the event, he said: "It was wonderful to see the Duke of Edinburgh with her."
The Queen and Prince Philip had appeared together on the steps of Buckingham Palace at 4pm this afternoon, when the National Anthem was played to mark the start of the garden party.
Despite the fierce sun, with temperatures at approximately 22C (72F), the Duke appeared relaxed and happily chatted to guests as he walked around the garden, giving no indication of his imminent admission to hospital.
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."
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 and yesterday toured a Victorian steamship, the SS Robin, in London's East End.
Earlier on Thursday 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.
The presentation took place at Buckingham Palace ahead of the garden party. The Queen is the Order's sovereign and additional members are appointed to commemorate important royal, state or national occasions.
Philip's citation lists his numerous military posts, patronages and honours associated with New Zealand. They range from his Duke of Edinburgh's Hillary Award, established in New Zealand in 1963, which encourages and motivates youngsters to become involved in a programme of self-development activities, to his patronage of organisations like the Royal New Zealand Yacht Squadron.
Police officers stood guard outside the central London hospital where Philip is to undergo the operation tomorrow.
The operation will take place on Friday, following the investigations that were carried out over the past week.
The Queen will still perform her engagements as planned tomorrow, which include a visit to the new BBC studios in central London.