The Duke of Edinburgh has been taken to hospital for treatment for a pre-existing condition, Buckingham Palace has said.
Prince Philip, 98, travelled from Sandringham, where he spends much of his time since retiring, to the King Edward VII Hospital in London on Friday morning as a precautionary measure on advice of his doctor.
He was not taken by ambulance and it was a planned admission.
The palace announcement came as the Queen arrived at the Norfolk estate for her Christmas break after carrying out her duties at the State of Opening of Parliament on Thursday.
The royal couple have been married for more than 70 years.
A Buckingham Palace statement said: "The Duke of Edinburgh travelled from Norfolk this morning to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition.
"The admission is a precautionary measure, on the advice of His Royal Highness' Doctor."
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The duke is expected to be in hospital for a few days.
Sky News' royal correspondent Rhiannon Mills said: "I understand that Prince Philip was not taken to the King Edward VII by ambulance and was able to walk into the hospital. It was a planned admission on advice of his doctor.
"The King Edward VII is the hospital that is always used by the royal family.
"The palace will not give details about the condition he's being treated for, but the decision to send him to hospital in London may be related to where his consultants are based.
"I understand that at the moment he is likely to stay in hospital for a few days.
"It is worth noting that the Queen has not changed her plans. She left Buckingham Palace this morning and travelled by train to Kings Lynn in Norfolk to begin her Christmas in Sandringham.
"She would have been travelling north with the knowledge that her husband was heading to London for treatment."
The prince has suffered ill health in recent years:
- 2008: Philip was admitted to hospital for a chest infection and was discharged three days later.
- 2011: Two days before Christmas, Philip was taken to hospital from Sandringham with chest pains. He had surgery and was discharged four days later.
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