A statement read: "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."
Queen Elizabeth has kicked off her Christmas break.
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Her Majesty made the trip to her estate in Sadringham despite husband Prince Philip being hospitalised.
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A source revealed to PEOPLE that the Duke did not travel by ambulance.
“It was a planned admission, and he walked into the hospital unaided,” the source claims.
A source revealed to PEOPLE that the Duke's hospitalisation is a planned admission.
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It is unclear as to whether the Duke will miss Christmas with the family.
It’s unclear how long Philip will remain in the hospital — or if he’ll miss Christmas with the family.
The Duke of Edinburgh retired from a life of royal duties in 2017 when he was 96 years of age.